First of all, you should not defragment if you are trying to solve real problems, i.e. anything besides "I have a lot of HDD accesses" because if there are logical or physical HDD issues, defragmenting can add to these. However, in your case, it doesnt even seem to matter, as you probably have a driver or a thermal issue, not anything related to your HDD.
Does it happen when running CPU-intensive jobs, like converting / editing video? Then its probably a hot CPU which is not adequately coolled - the fan could be blocked or faulty or the thermal paste on the CPU could have become brittle
Does it only happen in 3D games, and possibly only in certain 3D games at all?
Then its probably a bad driver update; there used to be one a week ago or two which was quite crappy, and you should uninstall that if you downloaded it, or download a newer version - maybe the GPU manufacturer found the bug and fixed it already.
It could be an overheating GPU, too. Same issue as hot CPU but its the GPU fan that doesnt work, or is blocked.
Hope that helps - for the Horde, and for great justice