Diodes, diodes, diodes!
If you have a shaker motor in the game, disable it in test mode. My girlfriends JP has an unreasonably powerful shaker motor, and it literally kills the diodes. All of the problems she had were diode related, which is good, because that is the easiest element to replace.
Usually, the diodes are the first things to go. Make sure the solder connections look good, and like thedefog says, check the switches. Maybe the outhole switch is being activated by ball 6 in the trough?
Quoted from Soapman:All drained and started ball 4, no ball kicked out, keep doing cycle. WTF!!
To me, this sounds like a diode issue on the kick-out solenoid.