I think you should clean and then oil your encoders. It sounds like they have a rough spot on them. While you're there, check out your o rings and look for any that are dry, cracked, stretched, or just don't look right. You can replace them all for about 3 dollars, so if in doubt..replace them, all at once.
While you're doing that, wipe down your tracks and make sure your wheels are clean and rolling smoothly.