Sounds like the tach sensor. If you're in conventional, it'll run somewhat normally. If you've shorted a wire, there's the amplifier on the top board of the PS32 stack that it pretty fragile and could be cooked.
I'd verify the wiring from the tach sensor and make SURE nothing is grounded or open. If not, I'd replace the tach sensor first, and swap out the PS32 top board next.
Obviously, I'm assuming the tach sensor spacing is correct and you have a good tach tape.