Some profiles can shake like crazy in flight. It could still be foaming, use real urethane foam to isolate the tank. http://www.shopatron.com/product/part_number=514/101.0 OS bubble-less clunks http://www3.towerhobbies.com/cgi-bin...&I=LXXF25&P=ML are amazing when used in conjunction with foam rubber. Poly foam(white) doesn't cut it. I'm seeing a lot of guys recycling the stuff that their home electronics came in, good for impact, but poor vibration dampening.

Are you using an APC 12.25X3.75? that seems to be the prop of choice for a 3-D profile.
Did you take the baffle out? That can cause a hideous flat spot at part throttle and out right kill it if you have other issues.
I don't like the "Power Box" muffler, if the baffle is removed, you gain an instant 1000 rpm, but at a cost.