Husqvarna TS138 (2015) Ride on mower, intermittent drive

[Andy88]

1st time this has occurred. Mower loses drive. Initially freewheel control lever had relocated itself to back position (freewheeling) while mowing. Reset. After more mowing it lost drive again. Freewheel lever won't hold back position but freewheels anyway. After some idle time, will reengage both forward and back positions, and drive. But not for long. It either drives or it doesn't. No creeping. No oil leaks, no grass buildup.
Any suggestions? Thanks,

 

[Craftsman Garage]

How is your drive belt?

 

[Andy88]

How is your drive belt?

Belts are all good. No grass buildup anywhere.
Drive seems to fail now after just a few minutes use, and always on a particular slope - mowing across a (gentle) left to right slope.. left side of mower is higher.
This morning no drive in that position, but drive when turned around, when right side of mower was higher. I'm thinking low transmission oil, or old oil.
Thoughts?
I'm about to pull it apart.
Thanks,