Have you changed anything in the muffler? If you have changed any baffles, it may be producing less pressure for the tank and then the fuel inlet is cavitating. Maybe it's not drawing air through a hole in the line, but either the stopper is loose and the tank is not pressurized, or the muffler pressure is too low and the inlet draw is separating the air in the fuel itself (pulling a vacuum on the fuel).
Something to think about. I also had a similar problem when a tree reached out and grabbed my nexstar. My good friend realized that the clunk slid forward got stuck with the silicone tubing kinked. This was drawing bubbles at high speed. I checked it by shaking tha plane and listening for the klunk, it was not there. Took the tank out and rapped it on the bottom and the clunk unstuck and now it is fine.

Good luck,
Curtis