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System keeps crashing with BSD and error Stop 0x00000050 or PAGE_FAULT_IN_NONPAGED_AREA

Have tried removing all but the necessary parts for sys to boot - tried swapping Ram and even hard drive. But same error keeps happening - reading on the net that usually related to defective RAM, be it main memory, L2 RAM cache, or video RAM have removed grphics card and tried different ram. Still same fault - this leaves L2 Ram as possible cause.
Is there a way to a) test it and/or b) replace it - or is it time to replace mobo or even whole PC?

As always any help and suggestions gratefully received.

TP