PC freezes when temps are normal but heat is on

my thermostat in my house for the time being is broken and can only be turned on or off, no setting a temp and expecting it to work. that's to be replaced at a later date. for the time being, when i'm gaming and the heat is on for too long the computer will crash. you might be thinking "well your temps are getting too high for the computer to handle and therefore crashes"... i thought of that, too. meanwhile i monitor the temps religiously when gaming and the cpu never goes higher than 120F in temp. i originally thought it was the GPU because it was an old GTX 970 so i bought a 1070 to replace it in hopes of fixing it. i thought it was fixed until it continued crashing while the heat was left on. same exact problem with two different GPUs? probably not the GPU. not too sure it's the CPU, either, for again, temp never exceeds 120F. what else could it be?? normally i'm good at troubleshooting but this one has me at a loss. thanks in advance for any and all of your answers!

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p.s. i had the issue before a full format and reinstall. ruled out software by formatting and same issue persists so can only assume it's a hardware issue.