My guess is fuel foaming. While the Avistar is a great plane, the inherent design flaw is the fuel tank. It is mounted directly attached to the firewall and very prone to vibrations thus causing the foaming.

Make sure your prop is balanced as perfectly as you can get it since an unbalanced prop will cause the most vibration. If that does not work you my have to make some modifications. If you pull the tank out of the hole in the firewall you will have to seal that hole up somehow to keep fuel from leaking inside. Wrap the tank in foam and secure it.

Good Luck!