unless the filter is only 1/2" tall or packed full of dirt I doubt that is the problem.

Rereading the problem it looks like it's a high load, minimal throttle situation where it occurs, I'm sorta thinking vacuum leak somewhere. WOT is usually rich anyway, so a vacuum leak doesn't show up as easily then. Generally, I'm thinking the ignition would not be that picky.

Have you done any vacuum leak tests?

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.