Over the years we have used the HS 84 tooth gears for many things, but I am wondering if there is a quality control issue. The gears, especially when meshed with a pinion, feel… well for lack of better terms… crunchy. It feels like it is warped or somebody put sand between the teeth. this has not been with one, but many. I have found a few that were out of round after testing.
Have you contacted Support with this issue? They may be able to provide you with replacement parts.
You can find the necessary contact information on this page: http://www.vexrobotics.com/vexedr/contact/contact