Ok I have a few ideas that haven't been mentioned yet.
1: No
terminator at the end of the chain.
I'd
call that my most likely suspect. The old techno's I used hated life without their
terminator and shook in what sounds like a similar manor.
2: Long run of crappy
DMX cable.
DMX is digital signal and therefore really shouldn't be bothered by noise (if someone wants to argue I'll post a link to a very thorough article on why this is true), but in an extreme case (long run, crappy cable, atmosphere with lots of noise causing things) you might have situations that there is data that gets messed up, though I would guess that this would be a quick jolt and a
restore almost immediately every once and a while, not shaking. Personally I'd guess a bad splitter before this scenario but that's already been mentioned.
Personally I don't buy it being
HVAC unless it was only one
I-cue that was doing it (thought process being
HVAC directly on that
unit could move it). If more than one had a shake but all the fixed lighting stayed put then I really don't think that the
I-cue on the end of the
fixture would cause it to start to shake while other fixtures stay put. If you have movement across all of the I-cues then it's hard to believe the airflow is direct and therefore it should
effect the other fixtures too.