is this a new problem that just developed?

Its not the vacuum advance. Simply put....the engine doesnt produce vacuum(or at least much more than maybe 1 inch) at full throttle. So any high rpm, full throttle issues wont be related to vac advance, because vac advance isnt in the mix or doing anything at all under full throttle.

Vac advance only works at idle/part throttle where the engine produces vacuum.

If its a new prob, Id be looking elsewhere