I own 99 gt since 05 and experienced those symptoms comin up on 180k replaced all sensor ( Maf, iac, o2 sensors). After replacing I ran a fuel injector cleaner that's when all the problems got worse ended up having to replace intake manifold gasket and full tune up. I thought problems were done but in idle it revs itself to 2k if gas is touched by a hair falling on gas pedal. From there get ur computer reset and it should be ok. Make sure u buy replacement thermostat and parts for all that were about $250 the work is easy just get the hidden bolts inside the corners of the manifold and put back on to torque spec and pattern in teardown manual. Any series 2 3800 motor is notorious for this problem had to do my bros 95 bonneville also so if u have this motor over 125k and have funny problems that would be my guess.
