Thanks Miggit, keps me puzzling and found this on the web: http://www.sonnax.com/articles/130-5l40e-5...rse-or-tcc
But can the problem be described as "loss of reverse"? It still goes reverse, although not as intended

The good thing is: oil looked and smelled normal and no excessive deposits in the pan, so re-builder concluded the symptoms are not ruining the box
But still....what is going on?

It sounds like there is a Hydraulic related problem in that box... some how it's cutting the oil flow to the reverse gear and then resetting it's self... so you end up with an on / off cycle.... Does it do it all the time or is it only under hard acceleration? which would increase the pressures in the system and possibly cause the over pressurisation... it could be a leek under pressure, but I still get the feeling that its' a pressure relief valve some where in the box that's at fault.

It might be worth asking an expert... say Sonnax? If anyone has the answer it should be them  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Thanks Miggit!
I'm going to talk to the re-builder with that and suggest he contacts sonnax. He already contacted the supplier of the valve block, but they thought it wouldn't be in the valve block and it seems they where right because a new valve block didn't cure the problem
It is indeed under pressure i.e. hard acceleration (on flat surface) or uphill and also when doing a stall test (it goes boink, boink, boink and you can't reach higher than 1400RPM). Under normal condition I always reverse at idle and then it's perfectly all right .