Is the clunk sticking to the back of the tank because of a slighlty long hose? It could also be stuck to the front of the tank from a hard landing or storage as well.

Also check on the ground to see if there are any bubbles running through the lines due to vibrations or air leaks. It really sounds like a fuel problem, not an engine problem to me.

Check the clunk for clogs or debris in the tank and make sure the tank is not moving inside the fuse.

Good Luck!

WOW, you guys are FAST!!