The CMS series mowers have an auto belt tensioner. I mowed many an acre over three seasons and never had to adjust.
The TZ varies from the bigger boomers in that it has a single lever throw to engage the PTO's that feels like a clutch arrangement on the other end. You can actually "feather" engagement with it.
While I don't have a fix, I'd have to wonder if there is some sort of operator issue causing this to happen twice? I mowed some pretty hefty stuff, stalling the deck more than once, which allows the guts to slip. I've also abused the tractor with a 5' tiller for a short time.
I don't mean to sound accusitory, but, I have to wonder.
What does the fluid look like?