My girlfriend has a 99 2gnt. Whenever she gets on it a bit (usually merging onto the interstate) the check engine light comes on and it refuses to shift above what I think is 2nd gear... possibly 3rd. It then stays on until she stops and restarts the car.A local shop said it was the computer but after replacing that it still has the problem. Open to any suggestion as the shop and I are out of suggestions.

My car did this shortly after a rebuilt tranny was installed, its called limp-mode or safe-mode. The car might be detecting a glitch in the elctrical system (in my case it was the speedo sensor going bad) and it goes into limp mode which forces the tranny to stay in 1st or 2nd gear.

Are there any other symptoms? Do the "keydance" when the check engine light is on to get the CEL codes. If you're not sure what I'm talking about, do a search of these forums for 'keydance' or 'CEL codes' and you should find what you're looking for, and that will give you a place to start looking.