91 K2500 speedometer
 

I have a 91 Chevy K2500, and it seems like whenever it rains and sometimes when it hasn't the speedometer acts up. When you turn the switch on it goes up to 10 or 15 mph and when you hit the starter it goes back to zero until it starts and it goes back up to 10 or 15. When you start driving it, it is about 10 or 15 above what its actually running too. I unpluged the speed sensor on the transfer case but it never moved off of it. When your driving with it acting up when you hit the brakes quickly it will get better and better. Its just getting to be a pain whenever it does this. Any help would be greatly appreciated. thanks