Repeating GPF

I've not solved the issue by contacting tech-support. It ended up with me asking the support guy how Netgear actually go about solving rare and intermittent faults in beta software, and I was pointed in direction of the beta firmware part of this forum.
My problem history so far:
No memory errors after a solid week of continuous memory test.
Tried factory reset on both my ultra 6 and pro 2 device, gpf's still happening every couple of weeks.
Tried going back to firmware 4.2.20 and 4.2.21 on ultra 6 device, no difference. I'm now running 4.2.20 on ultra 6 and 4.2.22 on the pro 2 device.
Tech support indicated that I had a couple of drives in the ultra 6 that was not on the "approved hardware"-list, but my ultra 6 did work fine for two years prior to the problems, so I don't believe that to be the problem. I haven't done any changes to the hardware.
Then techsupport mentioned that I could've transferred the problem through a factory reset by restoring my old configuration. I'm not very keen on doing another factory reset and having to configure everything from scratch. Nor am I keen on removing the disk drives that are not on the supported-list just because they are not on some list. The hardware did work fine for several years, even on same firmware version I'm running now. 4.2.20. The support case was closed without any real answers, That was my second support case on this issue. I've given up tech support for now...

Now I'm trying to change one setting at a time to see if it makes any difference... I've started by disabling any volume maintenance tasks. So far the pro 2 device has not experienced any problems for a month after I did this, maybe it is related. But the ultra 6 i still locking up every other week or so, even with any volume maintenance turned off. Next thing is to turn off the weekly backup jobs to see if that makes any difference...

Any input on how to debug this further/faster would be much appreciated!