If you feel happy about doing so, you could unplug the PC from the mains, open the case and re-seat the RAM and graphics card. The problem may be caused by bad contacts. This link gives a lot of information on doing both but in the context of building a PC click here
Failing that I can only offer a temporary fix click here and how `temporary it is I have no idea. As the words say the problem could be caused by heat or faulty memory.