I have a 98 grand prix gt....the rpm's would drop to 0 and the low traction light would come on....followed by something that felt like a knock in the engine...but the car usually would usually keep running...took it to the dealership where they changed an o2 sensor....ran fine for a day and then it started doing the same thing again....the car has been sitting for sometime now and now has trouble even starting...the tach will go from 1 rpm to .5 rpm and the back and forth and then the car stalls....other times it will run just fine for a few minutes and then the tach drops to 0, the traction light comes on, and it stalls....Anyone have any ideas...I had another mechanic tell me it could be the ECM...but I don't want to just be dumping money into it....would appreciate any suggestions

 

Have you checked to see if there are any Codes even if the CEL didn't come on ??
My car did something similar when the MAP sensor died. Good luck.

 

best source for these cars is clubgp (I used to have a 97 GTP). I swear I remember that the tach issue being something related to the crank or cam sensor. been a while since I've looked at these cars though...