Caltrac Tuning Info LOOK HERE

Looks to me like its bouncing on the sidewalls... I dont see much motion between the body and the diff. when it leaves. So my suspicion is its the flex in the side wall of the tire.
I would try a couple less pounds of air in the tire, and see that help the problem. I would go 9psi and work my way down to no more than 6psi.

Just my 2 cents