hmm so ur have onboard graphics...
maybe the cable has just gotten old and dodgy... but u say someone elses monitor started to do it aswell?
Motherboard problem im guessing then, ur best bet, as im not physically there to asses what PC u have/motherbaord type, take it to a local PC place and get a replacement motherboard, it doesnt sound that it would be that expensive *read another thread* as u have done that contract work so u could spare $30-$50.
