you need to put a spare/friends monitor on it for a while. It will eliminate, or not, all the PC-centric suggestions up above. Troubleshooting is binary chop, take away half the problem, repeat and so on.
Funny old story - there was a DataCenter in central London next to the Thames and 11am every Thursday morning all the computers reset. Engineers tried everything. One of the engineers was on a tour of HMS Belfast and the guide mentioned every Thursday at 11am they spun up the ships radar just to keep make sure it worked.....!
S