Tony,
Multiple OSes having reliablity problems running on the same hardware does suggest there is a hardware fault. I would proceed as follows:
1. Remove all PCI cards that are not required for boot.
If that solves the problem put half of them back etc. i.e. binary search which PCI card is causing the problem.
2. If that doesn't fix the problem and you have more than one memory module again try with half the memory installed, then the other half to test for faulty memory.
3. If you have a multimeter check the voltages the PSU is giving the motherboard are correct. I had a bout of random locking up when the CPUs 3.3V line was actually 4.4V.
Steve.