After hours of investigation I have figured a few things out. I was intrigued why the player switch only seemed to happen when the ball rolled over any one of the centre 9 rollover switches. I discovered that each of these cause the score motor in the cab to turn. The score motor has lots and lots of switches that are activated during a turn of the score motor - so that sort of makes sense. So the player advance is obviously being activated by one of those switchers, but it is not happening all the time - i.e. it doesn't happen every time the score motor turns. I then started to trace back the circuits using the schematics to see when things like the player number advance and also the ball trough kick out coil get triggered.
I have done a load more testing and then - hey presto - I saw something a bit odd. There is a bonus accumulator - that runs from 1,000 points up in 1.000 increments to 10,000. The current bonus is shown as a lamp lit on the middle of the playfield - 10 lamps 1,000 up to 10,000. As I was testing I noticed that sometimes this clocks down by 1,000. That seemed a little odd to me as usually on any game you clock the bonus up, but it never clocks down.
When a new ball starts the bonus starts at 1,000. Not sure if that is correct, should it start at 0? I checked the schematic again and figured out that actually it should start at 1,000 as their is a specific circuit on the new ball setup that advances the bonus counter unit wheel by one just before a new ball is ejected.
So, I started a new game and pressed various scores making sure I didn't press anything to advance the bonus from its starting point of 1,000. Eventually, as I suspected, something triggered the bonus to count down by 1 - therefore the bonus was at zero - none of the bonus lights lit. I then pressed one of the 1-9 rollover buttons, which triggered the score motor to cycle and BANG - there was the problem in it's fully glory - the player unit advances to the next player and the ball trough eject fires too. I confirmed this by looking at the schematic again - an sure enough as a part of the end of ball sequence the player unit advances and the ball ejects. This is only part of the end of ball sequence. Thee first part of it is to count down the bonus that has been scored - this is triggered by the ball being detected in the drain trough.
So, what's happening here is that because something is counting down the bonus from 1,000 to zero, the system thinks that is is part way through an end of ball cycle and so finishes that cycle off.
I did another test and clocked the bonus up to 3,000 and then pressed other scoring swithes until it ccounted down, then pressed the 1-9 rollovers. No problems were encountered until the bonus score went right down to zero, confirming what the schematic implies.
I then went on to find out what is bringing the bonus score down. It is every time the 1,000 score reel is triggered. It can be any players 1,000 score reel. So, that has now sent me on a search for a link between the 1,000 scoring mechanism and the bonus unit count down coil...........