Whats the cause?

[joedoh]

sounds like miss and detonation. your gas tank is full of rust and scale. its getting pulled up to the filter where the suction is blocking the flow and the engine starts to run lean being starved for fuel. when you turn it off or it dies, the suction goes away and the rust/scale falls off the filter material. start it up and its powerful again, until the suction plugs up the filter.

clean out your tank.