You didn't mention smoking issues, so one would hope to think it would be breather related. A fellow Tech got a similar unit for nearly free and found that the cleaning the breather system did the trick. Return hole starting to plug maybe? Of course using the correct oil would be a given. No signs of fuel in the oil? If not, you may want to check w/ Deere. There were bulletins on the smaller verticals for a similar issue I know, but I don't work enough on these really.