I am glad I saw this thread because I totally just solved this.
1. I cut my ceiling down and stiffened up the joists with 2X6's. This made it so my projector mount was screwed into solid wood in 3 places. This stiffened it up some. But the thing was still shaking!
2. Cut dowel rods to length to go between my projector and the ceiling, basically giving the projector support on all 4 corners. This worked and helped some, but the result was the LENS of the projector was shaking while the housing wasn't.
3. Thanks to the suggestion of a forum member, I cut a foam disk like a donut, compressed it pretty hard, and shoved it around the lens of the projector with a screwdriver. I have SVS too, so I basically just cut 2-3 inches off the end of a port blocker, cut the center out, and cut it like a "C". And then compressed this into the space of less than half an inch around the lens. This is what really solved the issue.
So, I guess I'd try # 3 first. I think it was the lens shaking inside of the projector. My projector has lens shift, so it is really made so that lens can move around. Doing this also solved my issue of the lens shifting when my son was jumping around upstairs too.
I think all of these things helped, but def # 3 is the one that sealed the deal. If you want pics of any of this I can take a shot of my setup when I get home. The dowel rod thing sounds funny, but it actually looks fine. I painted them black and put rubber ends on it. Makes the whole setup pretty solid.
Before I did this my dual PB12s were shaking my image up onto the ceiling that thing was moving so much. Now I can see a tiny bit of movement, but only when it hits really hard.