How does the engine run? How old is the distributor?

Check it with a timing light and see if that bounces around too. Try several diff spark plugs, and see how much the timing is "moving". See if it is a lot like what you see under your distributor cap.. both with or without advance.

Maybe this is way off target, but this "jumping" spark timing is what happens with Alfa Romeo distributors when the spark advance blades, pins, or slots have worn out, like over 100k miles.. They get all loose and let the timing "bounce" around excessively instead of a smooth even firing and advance curve. The car will usually run OK, the spark hits the right terminal, but the timing changes in diff situations - idle vs partial vs WOT - you lose power, and it gets hard to time it properly.

Cheers,
- Art