I would suspect misfire problems as when cylinder drops out at speed, low power/vibration are classic sign. Usual suspects are: faulty plug/plug wire (arcing?), intake manifold leak (doesn´t appear/not noticable till engine really gets breathing), clogged/obstructed jets, fuel feed, float bowl, etc. drags fuel/air ratio out of spec as flow demand increases. know this reply comes late but maybe somebody benefit. david, you out there?