Hadn't read this thread before, you wouldn't believe this Dave but I have a similar problem on my Cosmic Gunfight.
It has only happened twice, but sounds like the exact same fault.
Difference being my displays show commas even in player 3 and 4 which I was not using at the time.
The last time this happened the fuse blew on ball 2 just as it kicked out the ball.
I hadn't perceived it as a big problem as it had only happened twice and also I am using some crap fuses I got from Qld on ebay.

Did you end up getting quality fuses? I assume you have as it seems you have gone past that in your fault finding.

In the meantime if my CG plays up again I will watch a lot more closely.
Cheers,
Lindsay