Try using one stick of ram and testing each slot.
Then try a different stick of ram in each slot.
I have had screen display freezing and computer not responding problem at random times and I took my PC to the repair shop. They tested it for a few weeks and said that the GPU was broken (they said they had graphics issue in BIOS).
I had 2 year old AMD R9 280. They told me it is better to buy a new GPU so I ordered a new one for my self from Amazon (UK) and i have ASUS NVIDIA GeForce GTX 1060 6 GB ROG STRIX GAMING GDDR5 now.
Anyways right after I got my new GPU installed and got my case from the repair shop i noticed that the installed RAM went from 6GB (3x2) to 2GB! I opened up the case and removed/installed them back again and this time it was 4GB! I did the same thing and nothing happened it is still 4 GB lol. I have been using it for 3-4 weeks cuz I had important work to complete, but now i want to know what's wrong with this computer?