If you are going to be running a lot of PETG, you need to consider upgrading to an all-metal hotend. The PTFE liner inside the stock heat break starts to degrade around 230-240C, with higher temps accelerating it. Eventually it will degrade enough to cause the hotend to clog, whcih is often misdiagnosed as the extruder gear slipping. It can happen in as little as 1 spool. It’s entirely possible that you may already have this problem, or that the hotend clogged during the power surges you mentioned, preventing it from being able to push the plastic through at the increased speeds of the higher layers. You can read in-depth on the issue here: