whats your ignition setup? locked timing?
When I had an HEI crap cap dissy in my fiero my tach would jump quite a bit when vibrations got bad, idle and high rpm. after the 3rd pass it got worse so I looked and the tach output wire was loose on the dist, once I got back home I re-soldered it and it hasnt had a problem since. you might want to check the connections between your rev limiter (6al?) and your dist.
if you ran your best time to date with that problem I dont think it would be a mechanical problem