CASE 40 XT drive problem

I recently bought a used 40 XT 3581 hrs. I checked it out when buying it, but the area I was able it run it was a level gravel lot. When I got it home it ran like a champ at first. When the machine gets warm it looses power in its drive. I first noticed it when redoing the drive way. I was climbing out of the ditch and pushed forward on both control arms and nothing, the machine just sat there. There was no unusual noise from the drive motors the machine just refused to move. I had to rock it back and forth several times using only one control arm at a time, to get out of the ditch. Even hot it will run fine on level ground, but when warm and under load (both control arms being used) it just stops moving. I put it up for the night and returned the next morning it ran just fine plenty of power for about 20 minutes until the oil got warm then the same thing. Any ideas? I am thinking the tandem pump, but prying its not.