My workaround is to unplug and replug the mouse cord into the USB slot, which is easier than re-booting. I've been unable to detect any particular thing that triggers the mouse cursor madness. Sometimes I can go a day without it happening, but more usually, it happens five or six times a day. And of course it occurs most when I am most busy, grrr.
I'm thinking about reverting from El Capitan back to Yosemite, where I can work without the pesky cursor interruptions.
