Pretty hard to make a guess with so little info Eskimo.

Could be any thing from failing hardware to faulty drivers to corrupt/faulty FS9 add-on, etc., etc.

With more info maybe someone can help you find the solution:  System specs, how long has it been happening, any error messages, are there specific circumstance that cause it (same scenery location or aircraft, etc).

Will be watchin your thread.

Cheers,
Dave