Thank you for looking in to this. After the update, the 500MHz is consistently reproducible for me so far. I did have it happen on 3.05 prior but it was less common.
It seems that disconnecting the “problematic” charger (or dock), connecting the 180w charger and then reconnecting the problematic charger will reset this issue and the laptop will start performing properly, even with the other charger.
Here is a screen cap from amdgpu_top while it is in this state:
Now here is another undesirable issue, and I believe it is related to the above issue. When connected to my dock, if I disconnect the 180w charger, the dock connection gets interrupted every time. This means I lose all my external monitor connections along with any other devices connected to the dock, including mounted drives which are forcibly disconnected with everything else. I can plug the 180w charger back in and then disconnect it again to repeat this issue.