frankykeno is probibly right, its the monitor or the monitors built in power supply.

a refresh rate problem would be consistant requardless of the time or weather.

a pc power supply would cuase reboot problems , ie your system would reboot or lock up for no reason randomly.

how old is the monior adn what brand is it. ive found that non comercial grade moniors wear start haveing issues after 7 years or so. and refurbs if it was not new start after 5.