Same problem here. Surface Book 2 with i7-8650U. I've already returned it once to the local Microsoft store. I couldn't recreate it in the store (was very cold in there) but luckily I had taken video from my phone to show Task Manager dropping down to 0.4ghz during intensive PowerBI data-loads (it uses all available cores).
The replacement doesn't suffer from the problem as badly as the first one, but it will still throttle back to 0.4ghz during some long data loads. I have found that a box fan pointed at the back of the display seems to help as well.
Hoping this is just a firmware or driver update!