what would cause the engine to sure while accelerating up a hill between 2000-2500rpms yet not anytime else?

I'm looking into the EGR Valve/Catalytic Converter or Ignition Module, duct tape doesn't fix everything. It only fixes what you don't want to fix.

surge? up a hill, it tends to down shift the X-mission for climbing..... shake? a bad ignition module will shake on idle and run rough with power loss(miss) at 45 mph.

Check the motor mounts. The extra stress from the vehicle going uphill may be causing the slightly bad motor mount to act worse. Have someone watch the engine while the car is shifted from drive to reverse. Very little movement is allowed, with the double wishbone motor mounts in this the engine should hardly move. Like jason said any of those three would be present all the time not just uphill. A catalytic converter wouldn't just show up only uphill. EGR would trip a code and so would an ICM. If its not the mounts I would lean towards a trans issue. Its the added stress causing the problem so its something that doesn't do well with stress.

i have a grand prix and i am a mechanic. First many times i have had this problem whith mine. Four things this can be and i have had all four at same time lol..... number one, bad or loose plug wire and you have to understand the computer only logs a problem if the problem is in the computer for no less than one minute!!!!! The EGR , spark plug wires, transmission clutch lock out solenoid , Those are what it usually is with the grand prix's and the other that does not happen often at all is bad coil pack, or bad transmission torque converter or transmission.The top oxygen sensor can be loose and will also do it have that happen before on mine and it would mysteriously miss and jump and idle rough . My educated guess would be spark plug wire and or EGR if its not logged! it should eventually log the code it just takes a bit longer sometimes but if not check wires and EGR circuit those are what generaly happens and one or more of the others is possible I think you have pretty much everything that could do it!!!!! Hope you get it repaired cause these cars are fun to drive..........: ))

Transmission might be slipping. Check the condition of the fluid. You many need to change the transmission fluid and filter.

TPS ours surges at around 50 mph keeping your foot steady or on cruise control and mainly when going up a slight hill or incline .... i have ran tests with my modius and its pretty much leaning towards TPS .... if thats not it then i will have to hook up fuel pressure guage and watch pressures when it acts up , could be fuel pump starting to get weak ... car has 265000 miles and original fuel pump yet