RSEYP10KJV

We are looking at a 3 pipe Daikin K series R407c which seems to be changing mode every 15-20 minutes. it's like a defrost or an oil recovery . We think the outdoor fans slow but still run.
Sensors check out within range.We measured the charge as correct.

All the indoor units are on full heat.

The unit starts up normally and the gas pipe gets up to 50+c then all of a sudden it 'changes over' and the gas pipe goes cold. Doesn't last for more than about 5 minutes then goes back to 'normal' and the cycle starts again...

Has anyone got any pointers on how we go about finding the fault (we don't have a checker btw) and the rooms are simplified controllers

Thanks