Lumina '95 Overheating?

I have an overheating problem with my '95 Lumina. After some time of driving the temperature would go up to 3/4. I flushed the radiator, put new coolant and that didn't do anything. Then I put a new temperature sensor and I got all the engine power back. Probably the computer is convinced now that it is actually not overheating . The gage still goes up but comes down again, at least sometimes. Occasionally I also get a Service Engine soon but the Diagnostic System doesn't let me pull any code! Its simply not working. Is the PCM to blame? What can I do?

Did you replace the the sensor that interfaces with the pcm or just the one for the gage? There are two sensors on those one controls the gage and the other talks to the pcm. They are about 20.00 bucks from a dealer. Is the car actually overheating? 3.1 or 3.4 engine?

It's the 3.1 L engine and I replaced the sensor that dips into the coolant.
The car is not overheating since a friend of mine opened the radiator when the gage was in 3/4 and the coolant didn't splash out and the block was normal and not too hot. Do you know where the second sensor is? I'd like to fix it myself and I appreciate any help.

He should have got burned!!! Dont sound like the system is building any pressure.Thermostat upside down?Leaking in the motor?Bad radiator cap?

I already replaced the thermostat, though it wasn't faulty I think.
Another thing is that the fans are not being switched on either but they are working because I put jumper wires in the places of the relays and they ran. Maybe its even normal for that model that the gauge goes so high, I dunno.