I would look carefully for garbage in the fuel inlet, needles and fuel and exhaust nipples. Since it was running great it seems that the low speed needle setting wouldn't just go bad. Still, I would put all my money on a fuel tank issue as others have already said. Not just for leaks in the tank or plug, but perhaps a hole or cut in the inlet line attached to the clunk inside the tank. That's what it sounds like to me!
These small holes have given me fits in the past. Any air bubbles in the line at the carb during high throttle?