97 olds Cutlass supreme any ideas? Please help.

My 97 v6 160 hp olds has a little less then 130,000 miles on it. About 1,000 miles ago the check engine light came on. Then the car started to get bad throttle response. By this I mean you can put the petal mid way to the floor and nothing happens, you push it a little more and zoom rpms jump to 4000 from about 2000, car shifts down at five mainly because I let off the gas a little so I dont red line the engine then I slowly increase pressure applied to the gas petal and I get up to speed eventually. The car doesnt do this when I first start the car after its sat all night or for a while in a cool place. Then about the second stopsighn about 50 yards it starts. After sitting over night it has plenty of punch for a very limited time. Another thing that happens is when ever I turn on the fan the pitch of the engine changes to more of a wining noise. And the car does some sort of bucking action. There is also a tapping noise coming from the engine. And when I turn on the air/fan it blows out the dashboard vents for a while and then pressure is lost and it blows out at only the feet. I took it to a local garage they hooked it up to a diagnositic machine and it said the car was missing on all cylinders so they replaced the sparkplugs and all wires problem didnt get any better, I originally thought it was the transmission so I took it to the transmission doctors and they hooked the car up and said that the diagnoses was the same as before so I finally gave up and went to the GM retailor and asked them to take a look at it and they said I needed to replace my engine because they were not sure what was causing it and they garrunteed work done on the car and since they didnt know exactly what the problem was they didnt want to mess with it. O another thing if you lightly tap the accelerator it runs good up until about 2000 rpms in most gears. When the guys replaced my spark plugs the first time and it didnt fix the problem I gave them the go ahead to take the top part of the engine apart to see ifany gaskets were blown and things of that nature. They took it apart they said nope nothing wrong but when they put it back together they called me and said I had a broking ring. And I told them well it wasn't broken before you took it apart so that means that it was broken after the fact and you did it so you fix it. And so they did I think. And also the air filter has been changed and the oil filter and I always change my oil on or before its time. Could it be my timing chain?Another thing I noticed is the engine tends to be fine at spends above 65-70. And the problem became noticable a short while after I ran my gas tank to what I consider to be low (gas light was not on yet) so I put fuel injector cleaner in and I filled it up with premium and it didnt change much of anything except the weight of my checkbook. Speaking of gas the car tends to run alot better when the tank is full of gas, and the gas gauge stays at full for much longer then it should and once it starts to drop it appears that my gas mileage drops too. Alright know that you know absolutly everything I know how to explain could you hellp me?