Not likely to be HDD - scale is to 100KB - HDD capability would scale to 100MB.
What did you add to software just before problem started?
Which games have problem? Is that all games?
Get a utility which records history on temps and voltages and let that run in background while you play some 'problem' games... what are the ranges for these?
I would suspect the GPU first - last cleaned? replaced thermal paste?
CPU?...check and clean heatsink and all system fans... good videos on this on YouTube