Have your overclocked them? Mine did that when they were running too high (Overclocked, high temperature), they'd start showing artifacts, green lines and so on. Started running them at lower settings, and they were fine. If not, it's probably on its way out.
Search there must be softwares to test memory, if it dont pass it will show errors on which memory chip has defected you need to change memory chips. each memory will cost you around 14-20$ then you need solder.
Not just anybody can change memory chip, not everyone can handle soldering iron, it's bad advice telling a newbie to venture into this, better look for qualified personal to do that for you before making things more worse
He can also take it to a service/technician to get it repaired, I don't think he entirely meant to repair it himself.