Hi all, first post so forgive me if I am posting in the wrong place.
I have a 2002 Pontiac Bonneville SSEi, about 72k and i have noticed a potential problem. when the car is in 3rd gear and is going up a hill or i shift manually into 2nd, the car shifts hard as if hitting a speedbump but ONLY when it'* doing about 1500 rpm'*. This hard shift doesn't occur if i let off completely or give it more throttle than necessary.
does anybody have any ideas what this could be?

I'm kind of inclined to think the torque converter isn't coming out of lock up right away causing the hard downshift. The 4t65e is known for torque converter problems from time to time. When was that last time it was serviced? If the fluid is a little dark I suggest doing a service and using dextron 6 and see if it helps. Other than that your going to need someone with a scanner monitor the transmission when the problem occurres to help diagnose the issue.

Bad trans/frame mount will give the illusion that your trans is banging gears. When your on a flat surface and put the car into drive, does it BUMP?

Hey guys thanks for the reply'*.
William: the transmission has not been serviced to my knowledge (but it is going to happen very soon!) the fluid appears flawless and is the correct color, but I will change it regardless. thanks for the recommendations.
Mike: yes the car does 'thump' when put in gear after sitting, doesn't happen every time though. it is the same feeling as shifting into gear when parked on a hill. I also am thinking transmission mount, because it squeaks when first put in drive and also squeaks briefly under light acceleration but it'* not the suspension. Shifting up through the gears are firm and seem normal unless while the car is about to shift into 2nd and it'* given lots of gas-it jerks immediately as it shifts.
Is there a way to check the mounts or should I just take it to a shop? I am somewhat mechanically inclined but have never done these before.

Any chance you are having a mis-fire at higher speeds while the engine is under stress? I would also keep a failing coil, plug, or plug wire as possible issues.

Michael, I have owned the car for only a few months but have taken many long trips with it and have not noticed a misfire when it'* under stress; however the other day I noticed that on one of the spark plug wires there are a couple chunks out of the outside coating like it has been chewed on or something of the sort. The car seems to idle and run smoothly as far as I can tell so could this hard shift still possibly be just a bad wire?

Steve, Welcome to the forum~!
Really I should leave this to the gear heads but..I know with hood open on the drivers side rear u can see the mount and if u pulse the gas while in drive, with foot on brake, while someone is up front observing from the side, it should be solid without cracks and not more than an inch of movement aprox.
If the mounts are solid, I wonder if a P1811 long adapt shift code, could also cause this.
Last time my PA was on a lift, I checked the frame bushings and made sure bolts were tight and looked for cracks in frame bushings..all is well there on mine though.