MX700 mouse tends to occasionally "wander"

[Creig]

I've had this mouse ever since the MX700 first came out and haven't had any problems with it until a couple months ago. Occasionally I'll stop using it and the pointer will slowly track to the upper left until it comes to the upper left edge of the screen.

I've cleaned the LED emitter and sensor and it's sitting on an Xtrac optical mousepad, so it should be getting a clear signal.

Any ideas how to fix it?