list your complete system specs, including OS.
when did this start happening? are all of your cards firmly seated? check all of your connections (power, reset, as well as the monitor and PSU's AC cord) and make sure they are all plugged in all the way. make sure your MoBo isnt being shorted out on something (look at the screws and make sure they dont touch any of the MoBos traces). you do have the MoBo screwed to standoffs and not to the MoBo tray itself right?
yeah, these are all 'no duh' Qs, but you gotta start somewhere
