" when i do certain things on my pc - but not every time, my main display blanks and reports no signal "
Please explain what exactly you are doing when this happens.
If its when the machine is under stress,ie playing games,video editing etc... possible psu issue.
As you have built this system,check all cables are firmly in place,check the ram is clipped down correctly.
Though you may have a buggy driver installed,try updating or roll back the graphics driver.