I have a 98 GTP identical to yours. I accidently drove about 18 miles in 3rd gear.. (not paying attention) luckily i caught it in-time before serious damage, back to point since then my car has been doing the exact same thing. My boost will get stuck on 6-7 bars, RPM's jump to 5,000 + when getting into it to pass, had died a few times the day it ( 3rd gear accident happened) but thankfully stopped. When i initially clear the check engine light it runs fine again, except for the fact my boost indicator doesn't work at all...?? Until chk engine comes back on but then it is stuck at 6-7 bars!?!