The little micro explosions are most likely as some have said absorbed moisture that is boiling off.... most sites will tell you that PETG absorbs moisture quite well so if it's an older roll then it's likely to have a level of moisture.
As for temperature, you really need to test this for yourself on your printer, grab a temperature tower and print it out with varying temps.
I for example print transparent PETG @214c only going up to 220c max when i want it a little more fluid.
So at 214c i can extrude at 12mm cubed flow rate using a 0.6 nozzle, 0.4 layer, 0.8 width at 38mm/s, rock solid layer adhesion, no strings. If i have small holes <6mm then speed gets dropped to 32mm/s. I do run a high flow dragon hotend and can print faster but these setting provide good output, no blobs and at worst very fine hairs.