Your problem sounds mechanical to me. I'm thinking that you might have a dirty socket that the vid card is seating in, or that you could have a "cold solder joint" on the mother board, quite possibly one of the video card socket pins.  Also check the vid card itself I'd look specifically at the portion that plugs into the mother board.  If the pins appear at all discolored that could be the problem.  To clean the pins use a pink pearl eraser and just like removing pencil marks from paper rub the contatcts until they are bright and shiny.

That's about all that I can suggest.

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