[xlr8or]

Ditto on the fuel filter idea. When running hard the gas is flowing well through the filter and pushes all the junk up against the filter element slowing it down. Letting it sit allows the junk to settle and the fuel to flow again.
I had the same problem last month and when I pulled the fuel line and ran it into a can the fuel flow started out ok and went down to a dribble within a couple of minutes. Cleaned the filter and it flows just fine now. You may have to do this twice to get it all out.