Hi there Scotiwis
I just stumbled across your post/problem so thought I would share my experience. I have the XPS 730 H2C which gave me exactly the same issues. Random freezes and reboots, failing to fully boot etc. Nothing consistent and very annoying.
I cleaned everything and removed/replaced all key components but the problems persisted. At times I thought I had repaired it as it could go days without issue but then would fall over.
I eventually removed 2 of the memory modules and the problems went away. I therefore assumed that one of the sticks was at fault so bought 4 new 2GB sticks to replace the 4 old 1GB sticks (thought I may as well upgrade at the same time!). Again all seemed well for a few days and then problems started. I therefore removed a couple of sticks and so far it has been fine for a long time. I have therefore concluded that the problem is on the motherboard and relates to the RAM slots for the second pair (the pair closed to the rear of the unit).
I therefore recommend you remove and play around with the RAM modules and see if that solves your issue as intermittent faults are a right pain to track down.
I also got annoyed so just bought an XPS 8500 which so far seems great!
Good luck
David