Question Help with slow computer

DubleAX

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Dec 6, 2012
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Hello there
im sorry for my bad english and if this is not the good place to place this topic
im having a problem with a gaming pc lately
its 4 years since my parent brought it to me lately its running very slow
it has GTX 770 graphics card and i guess i5 cpu
and after running a game for less than 1 minute it loads very laggy [im playing right now no mans sky] and then it gets a black screen on computer like the monitor is waiting for hdmi port [the computer doesnt add nothing on the screen]
before 2 weeks ive given it to a technichian and he did clean up and installed free avast antivirus and cleaning the computer and some other things and it came back again 2 days ago after 2 weeks while it was running fun
i hope if i can get help here how to fix the problem