Don't know if I'd go back to that bike shop. That guy doesn't know much about bike design and geometry, I think. Speed might cause aerodynamic lift that would exacerbate a small problem in the front, but to say that every chassis would experience this is incorrect.
As to Monsters, I've never been more than an indicated 155 on one, so I can't say what happens when they go fast. However, I can say that I know of one S4RS is very stable at an indicated 155, even on rough pavement.
Tire cupping can cause shaking. Odd inputs at the bars can cause it, too, like Nate's experience above with loose jackets. Things added to one side or the other of the bars with significant aerodynamic drag (auxilliary mounts, in particular) can cause this. Mathematically, I suspect that gross misalignment of the front and rear wheels could cause this, but I've never seen that happen in the real world. I'm sure there are others, but I'd start by looking at the tires, then wheel alignment, then steering head bearing condition, then damper condition.
Or, you could install a steering damper to mask this rarely experienced problem. Your call.
I like the side mount ones 'cause they're cheap. If I crash, the condition of the damper when I pick the bike up is the least of my concerns.
Good luck, in any case. Headshakes at speed can be quite disconcerting.
