My car is an 89 Formula Firebird 305 TPI, 700r4, bone stock. When I first got the car, the speedo would jump wildly from 0mph to 10 mph over whatever speed I was at, like someone was just hitting a switch a just killing power to the speedo. Seemed to work better at higher speeds sometimes. Then after about a month it just worked all the time, just about 10 to 15 mph over the correct speed. My question is, what is causing this? Is is the VSS, speedo, or wiring? I also have a high idle when I come to a stop sometimes and have to blip the throttle to bring it back down. I head that was possibly VSS related. Any help would be appreciated.