T/B4 slipper or diff problem

[Racingdick]

Hi built my T4 over the last 2 days and ran it for a minute earlier to check everything,

The gearbox is slipping somewhere, not sure whether its the diff or the slipper?
what to look for in either case?

When building I did the diff to stock settings so that with a screwdriver in each half you could just about turn the gear, if that makes sence..

The slipper was done to factory too. even with the shaft and 3 turns thereafter..

any pointers would be appreciated as dont really want to tear down the gearbox unless I have too.

Rich