I've got the same issue too and it's driving nuts! Funny how mine also lurches more on incline and turning. Can't figure out what that is. I had a small gasket leak for a while and the issue started happening around the same time, coincidence or not?
I too was told not to service it as it may cause more problems but I eventually did. They replaced the gasket and filter and flushed the oil. The problem immediately got better but it's soon creeping back again. I've been quoted $6-9k to have it replaced or if I take it out myself and send it to ZF in Ingleburn they'll refurb it for $4.5k, still a lot of money.
Considering buying one from a wreckers but could be buying another dud as they're not tested let alone refurbed.
Has anyone attempted removing a gearbox?