[silicium]

It freezes when the graphics starts to work even for simple things like browser scroll or zoom, with any OS (Windows, Linux, FreeBSD) and either the supplied Quadro FX 3500 or a low-end Geforce 7300LE. Never freezes if kept in console mode with Linux or FreeBSD.
How to tell whether the PSU or the mainboard (voltage regulators) are defective ? The PSU is the original 800W model that came with the old mainboard revision and dual Opteron 2216. They were replaced with a second rev board and dual 8435, keeping the original 8GB RAM (passed MemTest without any error). Maybe the 1000W latest PSU rev is needed for these CPU's. There are not other heavy loads (only a 3.5" SATA2 or 2.5" SAS system disk and a PCI-E x1 USB3.0 host).

[johnnym]

It freezes when the graphics starts to work even for simple things like browser scroll or zoom, with any OS (Windows, Linux, FreeBSD) and either the supplied Quadro FX 3500 or a low-end Geforce 7300LE. Never freezes if kept in console mode with Linux or FreeBSD.
How to tell whether the PSU or the mainboard (voltage regulators) are defective ? The PSU is the original 800W model that came with the old mainboard revision and dual Opteron 2216. They were replaced with a second rev board and dual 8435, keeping the original 8GB RAM (passed MemTest without any error). Maybe the 1000W latest PSU rev is needed for these CPU's. There are not other heavy loads (only a 3.5" SATA2 or 2.5" SAS system disk and a PCI-E x1 USB3.0 host).

There were multiple revisions of the xw9400 system board and I always thought only the latest were capable of running hexa-core Opterons. Referencing https://blog.abcdef.be/2011/02/hp-xw9400...rd-hassle/, what revision do you actually have? I assume you will need to have at least -004.

Apart from that, did you already try to just use one 8435 Opteron for a test?