Considering that you reinstalled, etc - I'd look at the card. See if you can find a card you can borrow. You can try removing the card, blowing or vacuuming as much dust out of the case and card as you can. Then look closely at the contacts on the card - are they clean and shiny? If not, use a pencil eraser and lightly buff them. I don't recall if there's a fan on your card but if so, be sure it's turning.
If all checks out there, get a copy of Memtest (free) and check your memory, one stick at a time and see what happens.
another option is to do a clean boot - eliminate anything that is not necessary for system operation and/or FSX - and run it then - you want to see if there is some software running in the background that might be interfering.
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