Help with odd engine behavior

[Jfporco]

First a big thank you to the Pelican Brain trust. Lots of great advice.
Now, the car: 1975 Targa with Zenith carbs. I tried to go back to a CIS, but it didn't work, but that is another story.
Anyway, the car has developed a random event where the engine cuts out for just a mili-second (about the length of an eye blink). After the event happens, The tach jumps wildly and returns to the engine rpm. It has happened in every gear, at various RPMs, From 3,000 to 6,000. ( I do not have a rev limiter on the dizzy.). It may happen once, or 3 or 4 times in a row (about 30 seconds appart, like the engine power is immediately cut off). But it has happened with both a full tank and a half full tank.

On a 300 mile round trip, it happened about 100 miles into the trip, but just once. The car was traveling at 70 mph in 5th at 3,200 rpm. On the way back it happened about 8 times, same speed. Then this AM it happened at 4,000 rpm in 3rd. And it has happened in 2nd gear at 5,000, and 3,500 RPM. No pattern to the events.

I have a crane ignition with a crane coil, and this AM I replaced the coil with an MSD high vibration coil, to see if that made a difference....it Still happened.

I am stumped......it started happening about 2 weeks ago, but it doesnt seem to be getting worse (like something is going bad). I have done no work on the car in the past few weeks (no engine or electrical work, no wire or replacing fuses, nada!) Everything else is running fine. It idles at 950, backfires at 3,200 (as these zeniths tend to do). when the event happens the engine does NOT stumble or backfire. It is as if someone cut power to the engine for a millisecond, and as quickly the power is restored (but my pants are a bit more soiled!). HELP!!!

Is it possible that the crane is going bad? I don't know how old the unit is.
The alternator is charging but could the regulator be the issues? It was replaced 15 months ago.
Any advice would be appreciated.
Thank you again,
Cheers,
Jim

[fanaudical]

I would investigate the ignition trigger as well as every piece of wiring for the ignition and fuel pump. My first impression in reading your post is that you've got an intermittent electrical issue somewhere.

You might also look at your distributor and ensure bushings/bearings aren't worn and that there isn't a bunch of play that affects the ignition trigger (but I would think you would have more frequent events if it was mechanical).

You can always go back to points temporarily to see if the problem goes away.

[RSBob]

Check all your grounds

[Jfporco]

All wires to and from coil, dizzy, and grounds are secure. I am thinking it may be the crane ignition control since it looks pretty old. Anyone know if I can just swap out and MSD for the Crane? Thanks again for all the help.

[Targalid]

I had similar problems which cleared with a new distributor cap. Old cap had nearly invisible carbon tracks causing ignition crossfire at certain rpm. Your description sounds similar.

[Jfporco]

A happy conclusion to my odd engine behavior.
Targalid nailed it - Thank you!

The rotor and inside poles of the distributor cap were worn and appeared to be oxidized, a bit of 400 grit sandpaper and the car once again it terrorizing the neighborhood. A new rotary and distributor cap are on in route and will be installed shortly.

Thank you again for your suggestions and help.
Cheers,
Jim