PC randomly freezes while Gaming/Watching Videos with Flash

For a few weeks now my PC randomly freezes while playing games or watching vidoes (only videos requiring Flash Player so far).

It usually either occurs about 5 to 15 minutes after starting a game/video or not at all.
If I have already played for an hour I never experienced any problems after that point no matter how long I was going on.

Usually the screen freezes or turns black and I get a nasty sound loop.
USB devices don't receive power anymore and I have to manually restart the PC.

Sometimes (~5% of the cases) the PC freezes and gives a sound loop for a few seconds, then I get a blue screen telling me there was a system service exeption and it restarts on its own.

Sometimes the PC won't boot if I try to restart it directly after forcing it to shut down with the power button. The fans start spinning and it is making some noises but besides that nothing happens.
If I let it rest for a minute it will work again.

What I have tried so far:

- updating drivers
- checking for overheating (no anomalies)
- dusting the PC
- running a memory test (no errors while running it for a few hours, planning to do a 12 hour test tomorrow)
- fresh Windows installment

Nothing worked so far and I have no idea how to continue.

System specs:

Windows 10 Pro 64 Bit
AMD Phenom II X6 1090T
NVIDIA GeForce GTX 560 Ti
ASRock 870 Extreme3
Cougar A 450 Watt

~ 7 years old

Thank You for your help!

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The age isn't helping. But let's try this.

Google and disable the following.

Flash disable hardware acceleration.
Chrome disable hardware acceleration.

Flash is notorious for this and many machines locked up, bsod'd and more when Flash's default changed to hardware acceleration. The cause is well discussed so I won't get into that again.

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Post a link to this psu you have.

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