Oldsmobile/Buick Repair: Fluctuating RPM and overdrive hesitation, throttle position sensor, crankshaft position sensor

Question
Hi Van,
I have a 1990 buick regal custom with 3.8L engine. i recentely did a complete tune up and also replaced the oxygen sensor, brakepads, rear rotors, ignition, harmonic balancer, crankshaft position sensor, transmission filter and gasket due to leaks.

All this was done in a course of past 2 months. Now my problem is the engine RPM keeps fluctuating right from the second you place your foot on the gas and when the vechicle switches to overdrive, its hesitates and looses power but still runs, very annoying. Also if i go over 60mph, it runs fine.

I took it to pepboys for diagnosis, they could not find the problem and advised to clean the fuel system, which they charged $187, but that did not slove the problem either. I cleaned the throttle body and also put the GM top cleaner, nothing changed. I alredy have spent lot of money on this car beacuse i am a poor student and can not afford a used car either. any kind of advice with whats wrong with the car is greatly appreciated.
Thanks in advance,
Rob.

Answer
I would connect a fuel pressure tester, to make sure the pressure maintains while running.
If it does, a check of all vacuum lines, as well as the EGR valve,(don't replace it...clean it), and then check the throttle position sensor.(check it....don't replace it).

Van