sounds like timing problem if vacuum advance hooked to non timed port that means running at manifold vac, correct? means vac advanced all the way from the start as you throttle you loose vac and mechanical takes over. disconnect vacuum from dist completely and plug off, set base timing to 8adv and SLOWLY rev engine and watch timing change all the way till you hit your flat spot test drive and see if that helps:::: with vacuum still unhooked::::: if it helps try to hook timing to ported/timed vacuum