Tests - Switch Edge - bang on playfield - vibration set anything off ? if so adjust it.

If not, I'd start with basic switch matrix issues. Switch wired wrong, diode on backwards, a short to switch wiring or switch lugs. This will send phantom signals to your switch matrix.

LTG : )

Start with LTG advice as it does not sound like that board at all.

Did you play test the game before purchasing it? If so, did it seem to have this problem?

On No Fear, there is a feature adjustment (18) for a staged ball. If you set this to no, does the "random ball" added issue go away? If so, and the fact that you randomly start a mode, I would guess you are registering phantom hits of the center trough opto on the subway. When that opto registers a hit it will release the staged ball and if lit, start a mode (the previous ball in play then goes to the staged position). The staged ball was a feature to speed up gameplay, so there is always a 2nd ball ready to come up from the mountain. I actually disabled ball staging because even after cleaning the opto, I still got the occasional ball added. I suppose I should try to fix it properly as well