At one point, I had a pic. One of my vacuum chmbers was completely blown apart ..... look for goo comming out of it if you think it's busted. Doesn't have to explode to be broken, but that's a pretty clear sign if it is.

 

I wouldn't rule out bad spark plug wires. They can be intermittent and cause violent vibration/shaking problems. Try spraying some water on the wires at night and look for sparks jumping about. Any sparks and new wires are indicated.

Missing should set an engine code if it persists. Has CEL ever been on?

good luck

 

I would test the motor mount like JimBlake said and also do the super simple spark plug wire test TexasHonda suggested.

When I read title of the post, "motor mount" is what popped into my head, but if it only happens intermittently, I'm not too sure.

 

try a new cap and rotor and spark plug wires.... That did the trick for me.

 

Its cheaper to test and look at parts than buy stuff that might do nothing to improve the situation.

 

editing the post I made above on this post.... That engine BHG I swapped another engine in, changed my front motor mount and I have a different shake. It looks like the new engine has the stock wires still because it only has 46k Im going to swap the wires tomorrow with brand new bosch ones I purchased for the other engine (I have a lifetime warr on them so Ill swap them in teh store for new ones) and slap them on the fresh engine. Im also throwing iridium plugs in that bad baby so I need to.