timing unstable at high rpm
 

My timing is fine at idle and low RPMs, but jumps back and forth 5 or 10 degrees at 2000 - 3000 RPM. Timing is set at 4 degrees ATDC and now advances correctly to about 30 degrees BTDC at 3000 RPM.

I have eliminated the distributor as the cause by swapping it with a new one. I have tried both conventional points and a Crane electronic ignition. The engine is a Metric Motors rebuild with less than 1000 miles on it.

I am at a loss where else to look. Would a bad plug or cable cause timing problems such as this? What should I test?

Thanks,
Steve
1969 280SL

BTW - replacing the distributor (vac retard faulty) and adjusting the throttle linkage has fixed the engine surge I was previously experiencing - thanks for the help.