IC Diamond is the best, but expensive. Arctic Silver 5 is old, cheap, popular, and effective. I would suggest getting some Articlean
as well, it is cheap and makes removing old TIM extremely quick and easy, and removes more than 90% alcohol and hard scrubbing, and it's only $7 and you only use a few drops each time.
Underpowering a component can cause overheating, which usually happens with some backstreet communist Chinese off brand power supply that any second year American electrical engineering student could make. But with that PSU it's not a problem.
What are your ambient temps? Anything above about 75°F/24°C and you should look at a closed liquid cooler.
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