|Yes, about certain it is mechanical or a bad connection (likely).|
With the power off check all connections to ensure they are firmly home. With signal connections take them out and put them in again - same applies to SATA power and signal.
Clean all edge connectors, in particular RAM and any add-on Video cards.
If it still does it after that then while it is running gently move any cables with something insulated, like a small paint brush, while watching the screen. Using the brush end itself carefully on the board might make something show up, or even gentle tapping. The place that needs least touching or tapping to produce any change (good or bad) is close to where the problem lies. Be VERY careful because I probably shouldn't really be suggesting you do anything inside with the power on unless you are qualified to do so.
I am assuming it is a desktop/tower rather than a laptop.
Always pop back and let us know the outcome - thanks
message edited by Derek