Well I just called the shop after waiting patiently for 1.75 days.

Seems their tests are indicating memory (access or addressing) problems and the issue persists even with replacement RAM installed.
So the prime suspects now are:
1) motherboard L2 cache (or something else on MB);
2) CPU cache (an AMD X2 - 4400+ socket 939 installed new within 6 months).

Here's the hooker...
The motherboard is discontinued and cannot be ordered (ASUS A8V Deluxe) so I'm told that if it turns out to be the motherboard it has to go through the "RMA" process which sounds like it could take weeks!
They seem to be oblivious that it is my WORK SYSTEM.

I asked about an equivalent replacement and they are hedging on the issue.
Their main concern is that the RAIDed C: will be clobbered by any change in controller (I used the Promise controller on the ASUS (it has 2, VIA as well)).

Lessons learned:

1) WHen buying a new PC, buy an extra motherboard;
2) Don't bother with RAID, at least for C:. I gets you no benefit and lots of trouble.

cheers