How are you starting multiball?

If you start multiball from the back, you start with a number of extra balls equal to the number of solid lights on the center of the playfield (freezes). You can't do this until you have at least as many freezes as needed for that multiball (1 for fortress, 2 for museum, etc.). You can always get more freezes before you start multiball, to a limit of four, either via the standups/right ramp or via the computer awards.

If you start the multiball from the claw award (and already have the required number of freezes) you will get one more ball than required if possible. This gives you a three-ball fortress.

Clear?

Check your trough switches in test. Also, if you start a three-ball multiball do you get a three-ball multiball?

remember that the trough has a whole heap of switches in it--in this case I suspect that the second-to-last one (fourth? I forget how many balls are in DemoMan) is no good. When that ball is plunged it still registers that all-but-one ball are in play, so it kicks up another and plunges it. The next trough switch is good, so it quits there.

That may or may not be what's actually going on, of course, but is my first guess.

Not neccessarily--it depends which optos are bad.

OP: You most likely have a switch problem somewhere--use the diagnostics to check, rather than try to diagnose via gameplay.

If you insist on using gameplay: Does your multiball end when expected? (When you get down to one ball?)

Also, you may wish to change the title of this thread to try to attract game-fixin'-people, rather than rules dorks like me