I've been using a Ditch Witch trencher with front backhoe the last few weeks to dig up stumps and to knock poplar over. It worked great through the first 6-8 gallons of diesel. Once it got below 1/4 tank, it started to run really rough. It'd run ok at low throttle, but it wouldn't sustain a high throttle for long. I tried changing out the fuel filter. There was some orange "sludge" in the cavities below where the fuel filter sits. I cleaned that out. It ran good for another 30-45 minutes after a new fuel filter plus another 6 gallons of diesel. Then it started doing the same thing. I've taken the new filter off and there isn't any more additional sludge, and the fuel looks clear. I let it sit for a few days and went out again today. It ran good for 5-10 minutes, and then went back to being rough again. I have never worked with a diesel before, and I don't really know where to start. Any suggestions?