Recently I graduated into the double trapeeze skillset on my H16. With the increased speed however the rudder seems to want to pop up occasionally making the whole show get very messy very quickly. Does anyone have any tips on how to stop this?
Look at the bottom side of the rudder hardware, directly below the black rudder cam. There is a white plastic screw that can be tightened or loosened to make the cam rotate with more or less difficulty. In your case you'd want to tighten it a bit. 
The cam screw, as mentioned above, will tension a spring that loads the plunger against the cam. That screw is often frozen, so you may have to replace to make adjustments.
In the first place, make sure the system is locking correctly and that the cam is not always rotated in the down position, possibly not holding the rudder at all. Also, mis-adjustment of the newer systems is possible by moving the upper arm plate too far forward which can keep the cam form locking completely.
There is a FAQ on the Hobie Cat site on rudder cams...
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