Usually reloading software will take care of this, but you seem to have tried that already... can you manually downgrade to SP4? I am assuming SA isn't active and that is why you didn't go to 6.0?
I think only once I had a serious issue with this, reloading software never did finish it, and it turned out it was bad CompactFlash card... had to get a replacement from Mitel.
Probably worth opening a ticket with Mitel on this one if the system is down.
EDIT: Talked to another tech at coffee, he said it was a processor module and not the CF, but we couldn't remember the customer so we couldn't find the ticket to find out for sure.