Probably a TH-125 or 440-T4. They had the problem of a stuck throttle valve in the 90-94 grand prixs that would cause the car to upshift late or not at all. Or it could be a bad ground connection for the solenoid. Also a problem on 91-94.
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2000 GTP: 3" DP, PCM, TB shields, SLP Intake, Cat-Back Exhaust