ORAC,
You have the patience of a saint! One small suggestion:
If it's IRQ allocations that you think are causing the problems, go into the BIOS and change the setting for "Plug and play OS" to "NO" instead of "YES".
You'll have to map out the hardware positions and will probably have to resolve conflicts by hand, but maybe this will give a workable solution.
The only other thought is faulty RAM (Though Kingston has a good reputation.). Do you have/can you borrow another stick you can try out? Or will the shop test it? It sounds like the lock-ups are "random".
Good luck
Sq