I have an old (out of service now) 1.3 Ghz Athlon machine that I used for the main FS computer for a long time, and later as a Wideview client.
It used to do the same thing. Scared the crap out of me every time it did it too when the volume was up. It would always go to blackscreen with no mouse or keyboard function after it occured.
It did it with no third party scenery or aircraft. I tried a new sound card, that did nothing except to make the machine gunning noise sound quite good right before it died.

I took the case side off and sat a box fan directly in front of the case to help with heat, that did nothing either.
This computer was just a POS in my opinion. My son built it, and the components were nearly obsolete when he did.
It's now sitting on the bench in the shop while I decide what to do with it.
Sorry I can't really be of any help, but you aren't alone with the problem, and I never have figured out what was causing it.