What causes it to misfire and skip until the tachometer drops to zero?

indicates that the idle circuit or system is not operating as designed .
could be a sensor fault
best have the fault codes read to find the area of the problem

this could be the tach meter itself. there is no bad consequence to driving it this way just annoying.

Test ignition module.

Sounds like the tach is either bad and should be replaced or there is a short or loose connection to or from it. Check the wires in and out of it. If it's in the dash, though, it won't be easy to get to. Good luck.

What year is the vehicle ? The tach gauge probably is going out . GM has had insturment cluster gauge problem's . Faulty stepper motors .

ever do a real tuneup this decade
spark parts, and filters. that be first
and make sure the hot engine (sized not stated) is really hot.
180F or more, and holds, it must reach normal spec, temp.
or the ECU brain will not do its job right.
the only clue I have here is skips
skips must mean misfire.
there are over 50 causes of misfire.
bad spark plugs tops the list.
and all spark parts there, cap/rotor,wires set this OLD.
car is old , needs a major tuneup

they to forget all that other stuff, and elaborage
what skips means.
i guess never cold does it skip.

taCH DROPS TO ZERO BUT engine not heard to stall?
if yes, electrical problems or CKP failing?
what does engine do when TACH does that, matters big time.

Electrical arching in the ignition secondary circuit can cause this as well as a fault ignition module.

Possible failed fuel injector if the code is for one cylinder only. Multiple cylinder misfires can be caused by low fuel press. Another possibility is a vacuum leak at intake runners.

I have had the almost same problem twice. A 'thump', like the engine missed a fire and tach drops. Seems OK after restart. Electrical? Timing chain slip?
Has 135,000 mile. Any clues?

NOT SURE BUT IT KINDA SOUNDS LIKE EITHER A BAD GROUND TO THE POWERTRAIN CONTROL MODULE OF A BAD GOUND ON THE ALTERNATOR. ARE YOU HAVING ANY OTHER ELECTRICAL ISSUES OTHER THAN THIS?

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