Sounds like a fairly common problem with these cars. You torque converter may be locked up. You can simply disconnect the overdrive to prevent it from locking up if that it you problem. I had the same problem on my 91 Sunbird years back. The car's still running with the overdrive disabled, and there's been no problems with it (aside from reduced gas mileage).

1999 Red / Tan SVT #1969 of 2760 Built 4/21/99 K&N, DMD, FMS Wires, ES Insert / Bushings, PF End Links 17" FSVT 5-Spokes Factory Spoiler 35% Tint All Around