strange problem in light building !

i have a strange problem in ue4 (the last version)
when i build the lights, after 20% my monitor turns off and my graphic card funs work very fast !! and i hear strange sound so i fear and restart my pc !
i don’t know whats the problem !!!
my graphic driver is latest version, also i clear cache to test, but didn’t fix.

How long does it take to get to 20%, is it coincidentally what you have your screen save set to?