Hi all: I haven't started digging into this much yet, but searched and didn't find my exact symptoms. Here they are:

1) While driving along and with everything working fine, my tach will all of a sudden jump to much higher numbers and then move erratically.

2) At the same time that the tach act up, I'll notice some rough running - even at speed I can hear the car missing. Althought the engine starts running rough, it's obvious the tach readings are incorrect (jumping very quickly and reading 5000+ RPM when I'm in 4th gear at 50 MPH, for example).

3) If I come to a stop while this is happening, the idle is very rough and generally slow.

4) Here's the fun one: this can happen for a few minutes and then just stop and all will work great. Also, the last time it happened, I turned off the ignition, restarted and all was well (rebooted...).

Details: Stock 1975 280Z with 186K miles which has bene sitting for a few years up until the last two months.

My initial thoughts: Definitely electrical and something that can impact both the tach and the ignition. I think this must be a connection-type problem at the ignition control unit (ECU in modern terms, I suppose) or something else electrically 'global' like the voltage regulator or alternator.

My plan: Check all obvious connections to these three major components, then go from there if I don't find something loose.

Any thoughts or experience related to this? Does the tach get its reading via the control module? My car doesn't live at home, so I haven't had a chance to chase this down since it happend on Sunday; but I wanted to bounce this off the group...

check for any side to side movement of the rotor shaft inside the distributer.

Update...six years later. I never did get this solved with the stock engine, but it recurred with the stroker installed. It was traced to the ignition box in the passenger side footwell. A good condition replacement unit completely solved this problem.

Yes, it's an old, short thread, but I wanted to close the loop in case anyone else had similar problems.

You can also use an HEI to replace the stock box.

Now that's someone dedicated to following up on a post!