Hi.
I have similar problem.
I connected an external monitor (4k) with DisplayPort to MBP 15’’ mid 15 (m370x).
And I see that CPU speed is slow down.
I use Intel Power Gadget for monitoring.
The Cpu temperature never gets higher than 60 degrees, but Cpu speed easily can be stuck on 0.8Ghz for 20minutes or more. For the short period, it can work in the 1.5-2.5 range, but after that stuck again on 0.8Ghz. I thought I paid for 2.5Ghz + turbobust.
Now It gives me «1500 Multi-Core score» in geekbench and it’s impossible to work on this machine. Without external monitor 14000 scores.
It looks like the real bug.
What is going on? How to fix it?
PS: I also tried HDMI port and low resolution. It gives the same result.