I noticed my cameron micro drill press was acting funny after being unused for a while. It's paired with a fairly new model 2A speed controller. The problem was the motor would start to race for fractions of a second at low speeds. It would sometimes make stuttering sounds several times per second, even at no load, but only at low speeds.
Medium to high speeds were fine, and running it didn't help at all. The brushes looked fine.
I gave them a call about this and they said, yeah, we know about this problem. Some model 2A controllers were made with a bridge rectifier after the speed controller, which was supposed to reduce brush wear for 220volt market customers. For others, this caused the weird racing and stuttering effect. They said just remove the bridge rectifier or short it out and you'll be fine.
I did that today by just removing it to use somewhere else and things are OK again. So yeah, that's the official fix and it works fine.