My vehicle doesn't accelerate when the vehicle reaches speeds of 40 mp to 55 mp. Tachometer indicates increase in rpm, but the vehicle doesn't increase in speed.
This could be a transmission, plugged exhaust or an issue with the fuel/ignition system. Better get it looked at for a diagnosis, here are some shops in your area: http://repairpal.com/pontiac-repair-in-washington-district-of-columbia
RepairPal Certified shops can diagnose and repair your 1991 Pontiac Trans Sport. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.