Try a reboot. But it probably won't work. I've found that when the mouse starts jumping or 'disconnecting' (doesn't do anything until it's unplugged and replugged in) this means the mouse cord is failing. The wires aren't always making reliable connections anymore after being flexed for a few years.
In other words, time to buy a new mouse. You're just in time too, if you want a new Pro mouse. Apple has reportedly discontinued it, so if you wait any longer, they will no longer be availbele in stock anywhere.