2005 Pontiac Sunfire Please help Pontiac Sunfire issues

I took it to a shop down the road they put it on the computer and trouble code p0300 random misfire came up and the fuel pressure was around 20 so they replaced the fuel filter and went ahead and changed spark plugs. They called me and said the car was running good and ok to drive over the phone. When I went to pick it up they were closed and the car started but fan came on right away with engine stone cold and the a/c and defrost were off the car ran worse after they worked on it I didn't think I was going to make it the 2 miles home. I called them in the morning and told them it was still running bad and had no power. They said bring it back maybe they needed to reset the code again I told them I didn't now if it was safe to drive but they said it was ok the mechanic drove it before I picked it up and said it's fine. I m not even sure if they put it back on the comp but he came out and said I needed a new cat converter it was cherry red but they didn't have time to do it so they told me to go get a universal cat from advance auto and it would be good to go I'm a little unsure of why they didn't want to make the 600 dollars they said it would cost if they did it I would have waited until Monday I just want it fixed right. They said either punch the cat out or buy the universal one whichever I chose to do bring it back to them and they would reset the engine light. So my neighbor put the new one on he can do small repairs but he is not a mechanic but he put on the new cat which gave the car a little more power but that's it still running bad - cooling fan coming on right away and lacking power I'm not going back to those half ass mechanic down the road. I don't know what there issue is but I borrowed money to fix the car so I had the money to do it right but half has been spent screwing around with those jokers. I don't know what to do I'm afraid if I bring it somewhere else they won't be able to figure out the problem either and it will cost a lot more to narrow it down and I'll still have a broken car. I do appreciate your help answering my questions and I put my trust in the mechanic the fix my car I'm a single mom and I don't know crap about cars or how to fix them.

Do you think someone could have put something in my gas tank to cause this the mechanic said the fuel filter was black and sticky?

Yes the coolant sensor was the first thing replaced. Since my last post I also replaced the coil packs. Here is a list of all the parts replaced.
Engine coolant sensor
Coil packs
Cat converter
Spark plugs
Fuel filter
Thermostat
I replaced the spark plugs before the coil packs so when I went to replace coil pack I checked the new plugs again I noticed that the two middle spark plugs were perfectly clean and the two outside plugs were black. Does that mean the ones that were clean are not getting spark? These are the syptoms of the car again. First off the cooling fan cuts on as soon as I start the car even though it's winter and the engine is stone cold. The idle is rough and the engine is lacking power it's just running terrible I can't drive it. I just don't understand it was running perfect and I parked it over night and woke up to all these problems. I appreciate your help and if you have questions I will try to answer back right away I just don't know what to do and I've dumped a lot of money into it the last few days and no change. I could give u my e-mail or my phone number if it would be easier for you to contact but I understand if that's not aloud or to personal. Thanks again

Yes the fuel pressure was down when they put it on the scanner but after the fuel filter wAs changed it went up. They put it on the computer it got the trouble code p0300 which was random misfire. We have not checked the crank sensor. I was wondering if it could be the fuel injectors because we replaced spark plugs then decided to change the coil pack so we rechecked the spark plugs and the two plugs on the inside were still brand new but the two on the outside had black on them like maybe only two were getting spark.

I just don't know what else to check or replace because I've already replaced 5 or 6 things and I know it's difficult to help troubleshoot a vehicle problem over the internet

We replaced the crankshaft position sensor and it made no difference. My car does not use the camshaft sensor so that's one part I can check off the list. :)

I took the car to a friend who is a mechanic and he can't figure it out either. I don't understand how it could be this hard to figure it out. I'm not sure if he used a real time scanner but he hooked it up to some type of computer that said the car has a random misfire in going to check if he has a real time scanner. Since I wrote you last the guy cleaned the fuel injectors and put a fuel pressure regulator on it. The coolant seems to stay the same and I haven't noticed any white smoke. I have spent so much money and it's still not fixed. My friend/mechanic said they tested the cylinders and they all have even compression. I think he is about to give up on it to or put new parts on til he finds out what it is or I go broke. I do appreciate your help and I know you don't want me to go about it by buying a bunch of parts until I get the right one I'm just thinking which part will be the one that fixes it. Just a reminder so u don't have to go back and read old posts this is what has been replaced so far and in order of replacement.
Ect sensor
Thermostat
Fuel filter
Spark plugs
Cat converter
Coil packs
Crankshaft sensor
Fuel injectors
Fuel rail
Fuel pressure regulator
The car is lacking power and it's running to me like a sewing machine and the code from the obd is p0300 random misfire

I asked the guy working on car said it was a real time scanner and all it says is random misfire.

Oh by the way the radiator cooling fan comes on as soon as you start the car even on cold start and the a.C. And defrost are turned off.

He put it on the real time scanner it still only said random misfire. We replaced the ignition coil module and it is running good no misfire and the check engine light went off. I drove it today and it ran great but the check engine light is back on so I will bring it back and get it on the scanner again.

Oh by the way the car does not use a cam sensor