Did you check to see if the clunk is free? You can check by shaking the tank and hearing it bounce in it. Did you smack the plane on the ground on a not-so-smooth landing? I snagged a tree on an abnormally low approach a couple of weeks back and realized that the clunk slid forward and got stuck forward, soing the same thing. Also, it was dying after a few seconds just at WOT (wide open throttle).
Good luck,
Curtis