Many years ago I had a problem with a robot that somehow attracted a *lot* of dust onto it's IR sensors. It'd run for ~10 minutes, until the dust got too bad to work.
I had theories that static charge was responsible, but never managed to solve it.
My ghetto solution was to attach a high powered fan to it to blow dust away. See below image, its that orange thing in front.
Point being, maybe you can try various solutions to keep dust from getting into the encoder area . . .
