Not knowing whether this is a desktop or laptop makes for a couple of decisions.

I think that the problem lies with your power input.....but can be caused by many different things.

1) The first thing I would look for is dust within the computer as dust can cause overheating and cause shorting of the hardware that might cause arcing of the circuitry.

2) The power supply when old can also cause this type of issues. Because the power supply internals insulation get old and tend to get brittle and crack...this will allow hummidity to cause arcing when first started and when overheated as well.

3)check the motherboard very carefully for bulging filters...filters look like small metal cans and the tops and bottoms should be flat where the wires contact. They should be a bright metal (some have coverings ...labels) but dull, dark collorings of the tops shows signs of overheating.

4) This could also be caused by the monitor fixing to go out as well! If possible try using another monitor and see if the problem continues.

5) overclocking the video card will also cause this type of behaviour. Change the video card settings back to default and see if it still happens.

Good luck!