Could be as simple as needing to re-seat the CPU and Mem Sticks as with all the moving around and having had the sockets exposed could have led to a small amount of dust dirt or even thermal compond sometimes falling into the socket breaking the contact.
The random nature of the problem sometimes can be down to a problem like this which also has links to the slight movement in all connections due to heat build up. One sign of this would be if the system tends to boot up more easily after a longer time powered down or even when the reverse is true as leaving to cool down too much can also shift the contacts just enough to make a difference.
Just one more idea to add to the mix.
EDIT:
Carefully remove the items and very very carefuly check for obvious signs of bad contacts. I use a mini vac to clear any dust but often the process of inserting and removing a few times is sufficient to re-clean the contacts enough anyway.