Hello, I have a 99 Pontiac Grand AM it currently has about 200100 miles on it, everytime there's rain, the speedometer gauge keeps going up and down, the RPM's do the same, also the speed gets affected. The car seems to get stuck on 3rd and won't go more than 40MPH. Suddenly the car speeds up but it gets stuck again. I have replace the speed sensor on the transmission but it's still doing it, like I said, it only does it when it rains, after whatever dries out, the car works just fine. Any ideas? Thanks.
Does you car have Traction control? If so it sounds like a short in or around one of the sensors. Most cars with this option have a switch on the dash to temporarlly disable the traction control, try this next time to see if that helps pinpoint the problem.
If you donot have Traction Control. I don't have any other ideas.
I hope this will help you.