IMO, it's a vibration issue which does not show at first, due to a full tank. After some time, the fuel level lowers to a point at which the still present vibration causes excess leaning of the mixture, which leads to power loss/engine quit.

For starters, try a different prop, and if you are running a spinner, make sure it is in good shape and spinning true.

Good Luck!

PS: I have seen a tank foam up, like a Maytag clothes washer, due to bad engine bearings!