I'm having a problem when printing objects using white or yellow ABS filament where certain edges or spots will have burn marks. At least I think it's burning, as it looks to be brown or near black. I can't get it to reproduce reliably at the same spot on each print, but I'm pretty much guaranteed to get some burn marks somewhere on every print. In the picture, the burn marks are on a pretty steep overhang, but sometimes they show up on completely flat surfaces. I've tried lowering temps down to 210 (from a usual 235), but it still occurs. Any ideas on how to keep it from happening?

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