I have a JD 5520 tractor that has an apparent fuel problem that seems to recur. It will run normally most of the time, and suddenly start "sputtering" and losing power. Revving the engine will clear the problem and return to normal. It happens pretty frequently - like every 15 minutes or so. It seems that sudden uphill or rough terrain will make it worse. Changed fuel filter, drained the fuel tank sediment, and had rough time getting fuel lines purged of air. Got it going again and ran fine. Now it fails to start (after a couple of weeks of use). Tried purging air with no success. Anyone encounter this problem? Seems like system starves for fuel, but only at times. Any help would be appreciated.