OK, here's the results...
Ran the P95 Blend test, it errored after 58 minutes.
Did as Kickus suggested, lowered FSB to 166 so CPU:Mem now 1:1, ran P95 continuously and stopped it after 8 hours with no faults. Ran 3DMark05, no failures in the CPU test.
Tried oc'ing mem to 400MHz, no POST...