The thermal paste they used on the cpu dries and crackles over time, which means it isolates the cpu from the heatsink thermically and causes the cpu to overheat and trigger power down.
It takes 15 minutes at most to replace the thermal...
My 6805 started to overheat/turbofan/power off a few months ago and overtime the problem grew worse and worse to the point it would turn itself off while it was idle on a glass table in less than an hour... So I opened it and looked...