I should say that the stock RAM (20MBs) had been working perfectly well and then not. I have another batch of 20MBs as well, but none of those worked either. Tried my newer 4x 16MB SIMMs also. The MB looks clean as a whistle. Maybe I'll give it an isopropyl alcohol bath soon.

I've had the MB out about a dozen times trying to isolate the problem - it's clean underneath as well. It remains a mystery! I'll post a photo of the stock SIMMs in a few mins.