Comment by donut2d

So I had a thermal problem with my 2018 15" i7 MBP that sounds very similar to what you describe. Temps hit 60° and then get heavily throttled. It started happening after a repair to the keyboard. After dealing with that for 6 months I brought it in to Apple and they fixed the issue by replacing the mainboard, I think. Since it's very rare I see throttling for my use cases (compiling, some gaming).