First of all, Steve, did you put any lube in the cylinders for storage, or just let it sit? You might have some sticky valves from sitting, some minor crusting that will go away once the engine for awhile. Do you have a tranny behind the motor? an automatic? Could be the fluid "boiled" out and the torque converter is unbalanced. Or could be the carb is starving over a certain RPM, or the fuel pump is working at a minimum, and won't keep up with higher RPMs. lots of stuff it could be. I'd let the motor run for awhile, see if the problem works itself out. Good luck! Jerry