The sun tach in my car 1969 RR once in awhile jumps from like 2000 rpm to 8000 and then to zero and then back to the original rpms
what would cause this? coil,tach its self or something else,need help thanks there is no problem when this happens with the engine runing.

Highly likely it is the tach itself tho an intermittent open in the wiring could maybe cause that but I'd be inclined to think no. I would take it out & under the hood connect it up with some jumper wires and see if you could get it to act up then you'd know for sure it is the tach. I would also call or email Sun