while this might be off topic i think it could be related, i was having really bad stringing issues and needed to set the retraction to 7mm. but i was worried about clogging, so i continued to investigate and found that the thermistor is not secured into the heat block. i believed this was causing some inaccurate nozzle temps (as @Marek noted), so i applied a small amount of thermal compound into the hole. Since then i was able to lower my retraction considerably (down to 4mm). while stringing is not your issue, i would imagine that if the heat block is not holding a proper temperature it could also cause other flow issues (such as the grinding described earlier as well as clogging of the nozzle). maybe try it and see if it helps?