I've seen this problem on threads in the past but have not gotten a solid answer out of reading them. The machine runs fine for the first 20 minutes and then all sudden the left track will freeze up it'll go in reverse just fine just won't go forward on the left side only.
I had a 650K do the same thing when I worked at a dealership. I pulled the brake plate on that side and replace the o-rings on the brake pistons. I even pulled the final drive off that side and checked the planetary gears and never found anything I could put my finger on that was causing it. But after I reinstalled the final and brakes the problem went away. I'm not saying there wasn't something else but that's what I did.
We had an equipment company come out take a look at it ended up being too loose wires underneath the joystick that were not making a good enough contact and as they would heat up I guess they would expand or something and that's what it would fail. They ended up cleaning the connections crimping them making sure they had a solid connection we've only ran it for an hour at this point but it seems to be fixed.