As you probably already know it could be caused by any number of things including heat.
I would take off the side (the hot weather can be causing problems) and use a desk fan to blow air into the PC, if this helps then heat is the cause, check the fan on your GPU is working and all other fans too.
If it does not help then a System Restore would be worth a try (if your O/S supports System Restore) as it might be software related.
If S R doesn't help then I would use Device Manager to check for conflicts, look for yellow exclamation marks or question marks.
It could also be a Virus or too much Spyware, only scanning with updated apps will identify it.
Other possibilties are faulty PSU (substitution is the best way to identify it) or a loose or dirty connection, take connectors apart and push firmly back together (with the PC off).
Does your PC make any unusual noises ?
Do you have any warning signs ?
Do you have all the latest updates ?
One other thing which I have just read about is faulty components on the motherboard, click here and click here
A little more info would be helpful, which O/S, which updates, do you update automatically/regularly, have you made any changes recently to hardware or software, what is your hardware configuration and temp readings ??