I'll have to try this next time I'm filing... just a regular piece of chalk rubbed on the file or do you use chalk dust?Any and all times that I use a file, it gets chalked up first. That way, metal parings can't jam in the groove bottoms and cause the file to bump and skip. Of course, it makes the cleaning really easy, too.
Aluminum can be particularly bad for this. Next time, try chalking the file first. I like making aluminum fittings, the file work produces such nice smooth surfaces.