Gearbox 353 problem after refurbish

[Bravehart69]

Hello,

I have refurbished my 353 gearbox and have installed it in the rear axle. Now I have problem when the gearbox is in idle.
If I turn the primary shaft clockwise, everything is fine. Wheels are standing still like they should. When I turn the primary shaft counterclockwise after one or two turns it seems that one of the forward gears is engaged as the wheels are turning like when rear driving. If I wiggle a little bit I can go one or two turns and then it happens again.
What can be the reason for this phenomena?

Thanks
Robert