Hello,
I too have the same problem on a prusa clone with PETG.
making me crazy.
I didn't undergo all the tests @dant3 did but I did changed the entire extruder assembly and got few good prints with specific profile I made. since then I changed that profile and I'm back at the beginning.

I'm using prusa slicer if that helps.

If anyone has any suggestions,
thanks in advance,
Sagi.