There are several possibilities which come to mind. IF this a new problem, i.e. the engine used to work but now is displaying this loss of power, then I'd check the following:
i) Sometimes a small filter is installed in the intake tube in the tank. It would look like a stainless screen. That may get some particulate in the bottom of the tank sucked into it. The "crud" blocks the line because its being held by the vacuum and the engine stops. Then, after the engine is off for a while, the vacuum breaks, and the crud drops off, the engine starts, and and then the cycles starts over;
ii) You can get this as a symptom which happens always or inconsistently depending on the position of the pick-up tube, the nature of the crud or "gooh of life" in your tank, etc.
The first thing to do, is to take the pick-up tube out and check it. See if there's a filter there. It's possible that there is no filter, but there crud is "plate like" or has built-up so it's close to the bottom of the tube.
Ultimately, you should look into the tank and see what's going on in there. Do you have a Racor initial filter with a bowl? Does the stuff in the bottom fo the bowl shave some particulate? Is the filter picking-up "Stuff"?
