You have an intermittent problem, Matt, so things may not seen black and white.

If it were me, I would now be playing the game with the glass off, and when the fault occurs, grab the ball and simulate the problem. From what I understand, you are having a multiball commence before time. So drain all the balls and "simulate" the next ball - Once the ball hits the ball trough, depress any switches during its travel, and see what switch has triggered the multiball - if the opto in question awards points AND triggers the multiball - then we know it is DEFINITELY the opto.

The way the sensors work is that they detect a break in the light between the sender and receiver. If this path is obscured (by the ball) then the switch is triggered. Sometimes, a faulty opto can cause this "phantom" switch, or a poor solder connection on the boards. I have seen the actual LEDs have dry joints on the boards.

At this point - If you have isolated the problem to the optos, I would be taking them out and visually checking the soldering / wiring.

Intermittent problems are a pain, but to replace the LEDs is very cheap (if that is the issue)