PETG just is a pain in the ass sometimes. Really sensitive to moisture, and it loves to stick to hot metal. So it has a tendency to overextrude because of the steam, and bunch up on the nozzle, causing all sorts of havok.
The key to printing it is just keeping it dry -- the latest batches I've held feel like they're way softer than I remember, so I suspect mfgs are putting more glycol in it than before.
Also, do a sanity check and go back and print PLA from time to time. Sometimes you won't realize something else is wrong and you'll blame it on the filament, but something like the idler arm on the extruder is broken, etc.
You can print it on Textured PEI, or Glass - but I suggest putting a little glue stick down to act as a release agent on the PEI - PETG and PEI bond together too well in some instances (ESPECIALLY on smooth PEI)