GMC Repair: 99 Grand Prix transmission, tranny lines, pontiac grand prix

Question
Pete - I have a '99 Pontiac Grand Prix.  For some time now, during normal city driving, the temperature gauge will climb about 10 - 15 degrees and then my transmission begins to shift very hard.  Less frequently the temp goes up a bit more then I can hear the tranny slipping.  This has been intermittent for ~ 8 months but is getting more frequent.  I took it in to the dealer and they checked the cooling system and said they couldn't find any problem.  I changed the thermostat and had the radiator flushed but no help.

Answer
hello jim
this could be the rad. just the same thire is a tranny coller in the tranny . it is not colling it off anough if you look in the rad you will see where the tranny lines go in to the rad. the rad is the problem change it and you will be fine it is not part of the colling system it cools the tranny and will shift at certian tempt and by the compuiter . pete