HP Z800 lost half its ram

I have a HP Z800 with dual processors and 12 gig ram as issued directly from HP. It has 12x 1mb RAM modules and suddenly I lost 6mb.

I have removed and reseated all ram modules. I also removed both prosessors and replaced with new heatsink paste but no change.

I tested all RAM modules by swapping them around with no change.

Everything is pointing to dead slots on the MB those across Processor 1.

Any help would be greatly appreciated.