Your testing confirms that the GPU is involved but only as far as the PSU is too weak to support it, changing the thermal compound will change nothing I`m afraid.
You have two options, get your GPU tested in a computer that has an adequate power supply and borrow an adequately powered PSU to test the card in your PC.
Bottom line is that this computer needs a better PSU regardless of what you do.
You have two options, get your GPU tested in a computer that has an adequate power supply and borrow an adequately powered PSU to test the card in your PC.
Bottom line is that this computer needs a better PSU regardless of what you do.