Chevrolet Repair: 1996 Lumina Sedan acceleration problem, electric fuel pump, gauge needle

Question
Hi!  My car (1996 Lumina sedan 3.1l) started having problems accelerating both when shifting and maintaining speed on hills.  It acts like it is flooded and bogs down when you press the gas pedal.  Sometimes it overcomes it when I punch the pedal but not always.  I took it to my mechanic who hooked it up to see what was wrong with it.  He couldn't find out what the problem was as the sensors came back all okay.  He even test drove it but it didn't act up as bad.  He said it was probably just a tank of bad gas and said to put a gas treatment in it.  I did, but it didn't seem to solve the problem.
Also, it seems to be running hotter since the problem started.  The temperature gauge needle is well over the 1/2 way mark on the dash although not in the red zone. (That could be just from working harder with the problem and our 90 degree hot/humid weather spell.)  It does idle okay, and the problem seems to start after the car has warmed up.  Any suggestions of what may be causing it to do this? Thanks for your help!

Answer
It could take a few days for the gas treatment to work IF the problem is excessive condensation in the fuel tank that is fairly common.

It could be a fault with the thermostat that is causing severe variations of engine temp and should be replced as a precaution.

It could also be an intermittent fault in one of the sensors of which the scanner did not pick up due to the sensor operating OK at the time he tested it.

Could also be an intermittent fault in the electric fuel pump in the fuel tank as the pumps are well known for intermittent failures after 40,000 miles of which you should have the pump tested for pressure and volume of fuel.

Does the problem occur when the fuel tank is FULL of gas or does it occur when the fuel level gets below a quarter to and eigth of a tank ?
Give it a few days and let me know