I'd be tempted to pull the belts for a bit to rule out those accessories.
To echo what others have brought up, what are you using for a flywheel? I had been running a '360 balance' flywheel that turned out not to be, and same with my damper - the shake was unbearable though, and got worse as the speed increased. I did not see a shake that came and went, for what that's worth.
