Marshall JCM900 SL-X 2100 (5881 version) problem

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My JCM900 has following problem: capacitor C8 10000µF,16V and diodebridge BR3 (between V4 and V3 heaters) have been replaced due to malfunction. (the C8 had a bulb and BR3 was broken) The replaced BR3 immediately blew again after the first time back in action. When replaced by a bridge with higher voltage/current capacity, the amp started working again but each time after 20some minutes, and when the tubes really heaten up I witness short output errors. The output disappears for a short time (between 1 and 5 seconds) only to return directly afterwards and then again followed by the same error shortly later. The time between errors is not strictly periodic but happens each time unexpected. It seems like the BR3 bridge gets really hot, hotter than normal in my opinion. When VR1 and VR2 (pre-amp volume and gain sensitivity) are turned completely open, the error happens quicker. I was adviced to replace all pre-amp tubes (ECC83, 4 in total) but after having done this (all new ECC83's) I see no improvement. I'm pretty sure the error is in the pre-amp circuit but I can't find it. 3 other technicians, with tube amp repair experience have looked to it but neither of them could find the error.
Is there somebody here who has experienced the same problem, or can help me give some advice what to do next, any help is very welcome.
The three technicians that have looked to it were contacted by a local store and had all original Marshall schematics available. The schematics I have were taken from www.drtube.com . (I don't think there are major differences though. The pcb in my 900 is "SLXa-60-00".
The serial no. is 957554140.
If someone could help me but needs more info, please don't hesitate to contact me. Thanx.