When I look at PSUs I like to put the total Watts at double the entire system for long life. Here you have a GPU that all on its' own is close to half the PSU's new capacity.
Try this PC without the GPU as a test.
You can try to free up a few Watts by unplugging say the optical drive.
You can sniff out if it's a cooling issue by leaving the case cover off.
I'm not writing the PSU is the cause but it is highly suspect.

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