speedometer acting up on the truck. the tach, and everything else seams to be happy, but the speedo cant make up its mind some times. I don't think its a speed sensor because when the truck is stopped, it reads zero... but, when I stand on the brakes it jumps up to 10mph... extra voltage load is affecting the speed needle... any idea how to fix this? ive checked for codes, there are none ive reset the computer several times it is starting and driving just fine, it just cant give me accurate readings all of a sudden. anyone else have this issue on an OBD 1 truck?