99 Strat RPM Gage inop

My RPM needle pretty much sits on zero, ( both sides of the peg though ). On occassion the needle will bounce around to one side or the other. is there a sensor or is something wrong with the wiring, or the cluster? I've tried resetting the gages which will put the needle on the proper side of the peg, but after a few miles, the needle acts erratically. thoughts? Suggestions?