Grounds. :-) Look at the cam/crank input signal.

 

I'm running a MSPNP on a 92 1.6 with Toyota Cops. I believe my tach was off before I installed the MS. I've had a few issues, but I don't think the MS is the cause. While working on the car I noticed the two bolts that hold the gauge shroud over the steering wheel are gone. The leads me to believe that maybe the previous owner had gauge covers or swapped something. Based on other things I've noticed it wouldn't surprise me at all if the my car once had indiglo gauges .

 

More than a couple people have had issues with inputs - dig around and see if you can find one... You might clean that up. Check your logs, see if there are any jumps to obscenely high or low RPM. If so, there's something you should look at. I generally just look at max/min to see if there's any jumps, spikes to 0 you pretty much have to skim the log.