You're welcome. Corsair? - you should check the forums here.
Corsair has had lots of problems with its DDR2 RAM over the last few months, especially the 8500C5D (1066MHz). That is the over-clocked version of the 800MHz RAM like you have.
I am surprised that it passed memtest86+, did you let it complete all the tests? I would RMA it for replacement since it is not performing as expected. It should POST/boot reliably every time.