Hehe. Necroposting, but just found this thread again and thought I'd post the answer for my own benefit ...

Turns out the mobo is *very* fussy about DIMMs. Really fussy. Managed to get the machine stable with a single 512 DIMM in slot 0. Pretty much anything else doesn't work. To be fair, the mobo manual only recommends DIMMs that I either don't have or can't get.

Cheap PC -> Months of pain But, cheap is cheap, and I got there in the end

Al.