I have been noticing this problem with my 00 GP 3.8 NA coupe.
Here are the conditions: Driving between 35 and 75 mph, if I push the accelator pedal down 1/4 of the way to the floor, I will feel a transmission change/ studder. I heard a clunk! Not metallic sounding though.
First it will feel like it just locked the torque convertor because the car will feel like it' lurched even further forward and RPM drop too, and then will act like its lock-up choice was a mistake about a 1/2 second later and try to switch back. This problem then after a few seconds will often cause the engine to backfire once, and then I will let off of the gas and not go near that dreaded throttle position.
Everything else works fine on the tranny. I have fresh synthetic fluid in there and its filled to the correct level.
I did get a code that said "transmission incorect shift" or something like that. No Duh!
I looked at Rock Auto for a transmission solenoid, in case that's the problem.
I found one for $11. http://www.rockauto.com/catalog/moreinfo.php?pk=983089
But then I found one for $39, has something to do with pulse modulation: http://www.rockauto.com/catalog/moreinfo.php?pk=983068
Any ideas? I dunno if I am even on the right repair path yet.....