How do I keep the nozzle/hot end clean while printing PETG?

[Jesster]

So I'm a bit of newb to 3d printing. Just picked up an Anycubic Vyper, and after looking at my material options, decided to jump right in with PETG (I did buy one spool of PLA, but most of my prints so far have been using PETG). Mostly it's gone pretty well. I've played around with the settings a bit to reduce stringing and such. Generally when I print small figures or toys for the kids it turns out pretty well with maybe a few strings here and there that need to be trimmed off, but mostly things look pretty good. My one remaining problem seems to be printing flat, level surfaces. Whenever printing a fairly large flat surface I get a lot of flaky/feathery strings creating a rough surface and if I'm not watching it and manually trying to pick them out as it prints, they often bunch up in one spot and cause on ugly discolored blotch on the surface. After trying several things, I reasoned that it was just that the nozzle was getting to gummed up with excess filament that was sticking, stringing, and flaking off on the print. So I did a thorough cleaning of the nozzle and started a new print of a flat piece, and watched as the first few layers went down nice and clean. I came back a few hours later after it was done printing the infill, and found it had gotten all gummed up again, and I had the same sorts of problems when it printed the top layer. I'm using a silicone sock on the hot end to try to keep it clean, but that doesn't seem to help much with this issue. Is there anything else I can do to prevent this sort of build-up on the nozzle during printing? Some of my relevant cura settings: Layer Height: .2 mm Print Temp: 220 Build Plate Temp: 70 Print speed: 45 mm/s Initial Fan speed: 0% Fan Speed: 50% Retraction Dist: 7 mm Retraction Speed: 25 mm/s Travel Speed: 150 mm/s