Follow-up to Dual Athlon XP MP board problems
It appears - though it is still too early to tell - that my problems with the dual Athlon XP MP system has to do with something called "slew rate". There is zero documentation on this feature in the motherboard manual, and setting it to "auto" is what was causing the problem.
Apparently for XP processors, the "slew rate" must be set to "1". So far this has cleaned up the stability problems, and the 2.4.19.x kernel is supporting the hardware in the box. Speaking of 2.4.19, I'm looking forward eagerly for it to be nailed together.