20 seconds is not too long for something to overheat. The hottest components, like the CPU, can reach maximum heat in seconds, and if their thermal solutions aren't working correctly (apparently your TC reapplication was not a success) can overheat to the point of shutdown very, very quickly.
I would say that overheating is almost definitely your issue.
So what steps should i take next? Attempt to clear the CPU fan of dust and make sure its properly seated? Maybe even reapply some thermal paste?