Are you near the printer when it happens or in bed? You really need to be there so you can get some idea of what is going on, otherwise it becomes a bit like looking for a needle...
Is the filament sticking in the reel perhaps.
Can you post your speed/extruder temp/retraction settings. What makes it strange is that you are seeing no grinding on the filament. If the filament is meeting resistance then one would expect the drive wheel to cut into the filament and leave evidence!
Does the geometry change significantly where the problem occurs - a pic might help us.
Maybe the filament is odd and does not have a uniform diameter of 2.85mm.
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