Hi, I have a 93' SC400, and Sometimes lately, (althought it hasn't happened recently, and it has only happened about two or three times) I start the car, and get going, and I realize that the speedometer stays at zero, why is this? However most of the time it just jumps right up the speed at which Iam traveling. what could the cause of this be? And what should I do? Some advice would be great, thanks!!!!