Disconnect tach and take it for a test run, gotta eliminate the tach... I also had a problem once on a customers boat where corrosion was in the harness that plugs into the ign. module...

Also, put your timing light on the ignition coil. Turn on your timing light when the motor starts breaking up and look at the light, if the light breaks up you know it's iginition... You could also have a bad coil wire, swap from side to side.

Do one thing at a time so you can find the problem.

what about the module in the dist. and I have also seen the tach's cause similar problems, these have already been mentioned, but they are good possibillities.

I would definitely check the tach. Had the somewhat similiar issue with mine. Told my mechanic to check it out but he forgot. Would only go to 4000 RPM's. I didn't have a preformance issue but when I went far enough past 4000 RPM's it burned up my new igniton system.

We had the same problem and found out one of the new fuel pumps was bad and lose pressure at 4,000 rpms. Found out by putting pressure gauge on fuel line at carburetor. It had 8lbs pressure at idle and as we accellerated it kept dropping.when we reached 4,000 rpms we had 1 lb pressure.Put new and no more problems.

Check Engine Grounds, Fuel Pressure, and inspect 10 pin cannon plug. It will be one or a combination of the three I am betting.

An ignition module failing in the distributor is the most likely cause. Cheap fix. As others have mentioned a faulty tach can also do it.

I'm thinking it's electrical or ignition related and nothing to do with fuel because he said the tach was jumping from 3 - 8 k.

That's where I was getting ready to go with this. Had it happen before on a race car.

It takes less time to disconnect the tach then swapping it. Just use your ears to control the RPMS instead of your eyes while you're testing. You can also hook up a digital snap on timing light while you're running, I've done it many times.

Take the grey wire off the back of the tach, or diconnect the grey wire from the ignition coil, tape it up and go for a spin. Should take no longer then 15 seconds.....

I'm with Panther on that...good luck, let us know what you find.