I too had a similar problem, it turned out to be that the ethanol gas had loosened the varnish that had built up in the fuel tank. As the engine ran the debris would build up around the strainer in the tank and choke off the fuel flow. Shut down the engine and it would drop clear and all work fine for awhile. I had to empty the tank and flush it clean, replace all filters and hoses. Then run a tank of gas with carburetor cleaner in it to finish the job.