I must thank you guys once more...
Had the car out on the track today, and boy, does it flies

.
I had a problem with the diff once more after a 2 minute run, seemed the lock nut had been broken. Heard from someone that putting an extra 2mm, no-lock nut in first and then the lock nut, would greatly enhance the stability of the diff (no need to worry anymore about damaging the lock nut), since the force is now distributed over two nut instead of one. So I rebuild my diff using this methode and I must say, you can really thight it down now. (locking it fully and turning back a 1/4 of a turn)
Next I found out that I had set the slipper to tight. After loosing it up, the fun began

. This is the first time I really felt what the car is able to, and I must say I love it

.
I do hope that people stuggling with the same probs I had, can get some usefull info in this theard, thanks to the help from all you guys.