Hi John,
The TS125/150 motors are prone to clutch drag even when otherwise in perfect condition. When cold they will often want to jump back into neutral and try to drive the bike when in gear even with the clutch fully disengaged. Try running with some thinnish oil in the gearbox for a while. There’s nothing special in the box that needs heavy grade gear oil so it won’t do any harm. I run mine with SAE30 mono bought for the motor mower and 20/50 would probably be ok.
PeterF