One other thing you might want to check - just in case - is the state of the capacitors on your motherboard. If you have failing caps, you can get all sorts of odd behaviour, especially when a machine is first started. I saw one machine that would only detect the first DIMM when first switched on, but would detect all 3 fitted DIMMs once reset.
gives a couple of good pictures that show what to look for. These problem capactitors may have first appeared years ago, but they still pop up from time to time.
Of course, this may have absolutely norhing to do with your problem, but it is worth checking anyway - best to be aware of a developing problem before it gets too serious.