I wanted to add a quick follow-up to my situation. My situation was that as soon as I plugged in an external monitor, Radeon High Side voltage went up to 20V, temperatures shot up, fans went to 5300, and kernel_task soon throttled the CPU completely.
I visited an independent repair shop who cleaned out the fans and internals of the case. When I plug into an external monitor now, the temperatures rise a bit, and fans get up to 3700, but the temperature remains low enough (150F) that kernel_task doesn't kick in.
So by all means, it still seems like there is a serious issue here if some dust in the fans can cause kernel_task to throttle the CPU to nothing. And I'm not thrilled that the fans need to run at 3700 as soon as I plug in an external monitor. But if you haven't cleaned out the fans and internals of the case, give that a try.
I visited an independent repair shop who cleaned out the fans and internals of the case. When I plug into an external monitor now, the temperatures rise a bit, and fans get up to 3700, but the temperature remains low enough (150F) that kernel_task doesn't kick in.
So by all means, it still seems like there is a serious issue here if some dust in the fans can cause kernel_task to throttle the CPU to nothing. And I'm not thrilled that the fans need to run at 3700 as soon as I plug in an external monitor. But if you haven't cleaned out the fans and internals of the case, give that a try.