The switch on the shifter is pretty simple, one wire goes to the pcm and the other wire goes to ground. If the switch is dirty and the button doesnt work freely then this could cause your problem. More involving..... the signal wire to the pcm anywhere between the pcm and the shifter button could have rubbed through the insulation and is intermittantly shorting to ground and causing your problem. There is a pigtail on the shifter with a two wire connecter and you could try disconnecting this and seeing if the problem dissappears but you also lose using performance shift to try this. You could also get another shift knob from a 98-00 GTP and see if that fixes it.
