Cooling issue and running rich?

In the past few months, I have done a lot of needed work for my vehicle listed above Express 3.6l V6. I had a check engine light come on and off for several months after buying the car used from a lot. 1 code was on, PO301 (Cylinder 1 Misfire)
I started out with changing out the spark plugs because they have never been changed. I got all new plugs and changed them out, cleared the code and still came back on, changed out the coil packs and it cleared the code itself and the problem went away.
since I got the car, it has had a small cooling issue. If I accelerate in anyway my coolant gauge sensor will go all the way up to overheating and then if I let off the gas it will go back down to running temperature in a few seconds. Never stays overheated or hot, fan will come on and off when the needle raises and lowers. I did a cooling system flush and did not fix or help it at all, bought a coolant temperature sensor to replace the old one, can't find the sensors anywhere to change it.
flash forward to recently, I thought maybe my fuel injectors were bad, I get horrible gas milage, I have a bad odor coming from my exhaust, and sometimes it spits and spudders when I drive and then goes away when I let off the gas for a few seconds. I got 6 new injectors, took out the old put in the new, dipped each in gas before inserting them back into the fuel rail and engine. Car ran good for about 2 hours, and then check engine light came on for PO712, running rich bank 1.
i don't know what else to do about the cooling issue or it is running rich, took it to a shop for both and nobody can figure it out.
I got 6 new ones, and put them in myself, seemed simple enough to do.