there 2 issues that are most common causes of things like this.
1) is a faulty ram stick. simplest way to check that if you have more than 1 stick is to remove one and then see if the issue occurs. if it does, replace it and remove the other one. if that cures it, you have found the issue, if it doesnt, try cleaning the ram with a can of compressed air as well as the ram slots. and test again.
2) if none of that works then its a gpu fault, again more thsn likely faulty ram on the gpu. heat could cause artifacts so check gpu temperatures.