Had exactly the same issue with my daughters new PC a few days ago after a house move; turned out it was simply down to the memory stick being slightly loose. Would suggest that you make sure that your memory stick(s) are firmly in place as this would produce exactly the problem you are describing.
Only just managed to use another PC to update this.Have tried re seating memory. Graphics were built into mobo, tried new graphics card. Tried connecting with different cable, tried another monitor- still u/s! Is there anything with th PC itself that could cause the problem?? Running win7, 2 gig ram. Thanks