Losi 8ight notchy gearbox

[gps3300]

Rear gearbox on my 8ight gets really tight & almost locks up at one particular point when turned by hand.

If I back off the LHS gearbox clamping screws off a few turns it frees up

Can't see anything wrong with the gear teeth on the crown or the pinion gears even on close inspection.

Gears are almost new, no wear. Car has had approx 10 runs. All I can think is the diff crown wheel is doesn't screw down evenly onto the diff casing making a "high" point? This gearbox is particularly noisy.

Anyone else ever had a similar problem?