I have a 03 g.p gt about 155k recently my trans went out on me and I replaced it with a trans from a 01 Monte Carlo couple of plugs we different but I was told the trans would work after I installed the trans there was no noticeable problems then out the blue it had a problem shifting out of second like it won't shift until my rpm hit 3500 which is not the best on my gas mileage everything seems fine I was told that the trans from the 01 might not be fully compatible with my 03 computer and might have to be re wired or could this just be a internal part in the trans going bad

If you had no noticeable problems, then I think you have a problem with your "new" transmission....

Did you scan for codes before you put in the new tranny? Have you scanned for codes after putting in the new tranny?

You don't say what happened to your old tranny....

Also, did you flush the trans cooler and it'* lines before connecting the "new" tranny? Did you empty the converter and replace the filter on the "new" tranny as well as installing new seals(converter, drive axles, etc.)?

With 4T65E boneyard trannies, you should figure in the cost of a new valve body w/solenoids, so that you don't inherit the typical problems(pressure control solenoid, TCC solenoid, shift solenoid, etc.)......much easier to install, before putting the tranny into the vehicle...

Does your GP have a SC'er?