Daewoo DSL 801 Drive Problem

Hey guys, just figured out how to post on this site after over a year! I bought a 97 DSL 801 about a year and a half ago, and have worked on it ever since The problem is in the drive system. After I use it for a half hour or an hour, the drive power starts to fade, and if I keep running it it will not even have enough power to move with a bucket of dirt over any uneven ground. I had a local tech from caterpillar put guages on it, and we figured out that the drive motors were worn out and the case drain lines were bypassing most of the oil to the tank. I replaced both drive motors, but to my dissapointment, it really didn't make much difference. Still loses power after operating for 30 minutes or so. The hydrostatic pump looks brand new, and was supposedly replaced just before I bought it. The filter in the resevoir was replaced at the time I did the motors last winter, and oil changed, tank cleaned. Hyd filter changed 3 times since just to be sure. Oil cooler flow checked, and lastly I checked temps on the inlet and outlet of the cooler (140 in, 105 out) and checked temps on all 4 hydrostatic pump relief valves to see if a fluid bypass was creating too much heat, but all 4 the same 140-145 deg F. Only thing I can think is that the charge pump is putting air into the oil from a bad seal, but all other functions work ok when drives get weak. Did notice that snowblower running on Aux hydraulics seems to get weak too as machine heats up. Not sure how to proceed. Hydrostatic pump showed 3,500 psi with hoses off drive motors, and service manual says to replace/repair if under 4,000, but service tech said I only was using 1,000 psi with the bad drive motors, and 3,500 should be fine. Hate to replace the brand new looking hydrostatic pump to find out it was something else. At this point, anything is going to be big $$$, but it would be well worth it if it would work for me. Any help would be greatly appreciated.