Hi deerparkfacilities,
The problem you have is caused by warped brush tubes.Generally this is caused by the motor getting a little too hot and causing the brush tube housing to heat up and expand to the point that it distorts the tubes enough to not allow proper clearance between the brush and brush tubes.
This can be fixed in one of two ways. You can replace the brush tube holders on the motor or you can take a sheet of 600 sand paper and lay it on a flat hard surface then carefully take your brush and remove a few thousands off the sides of the brushes.This will give a little more clearance between the brush and tube.This should fix your problem.Hope this helps.Good luck.
Tinker