Just a thought. Have you inspected the trough itself for indentations that the balls can hang-up on? Common problem. There is a permanent fix by filing flat Vs. various commercial and home-made plastic insert sleeves to remedy the issue e.g. http://www.pinbits.com/index.php?main_page=product_info&products_id=449

If you have some fairly thin plastic, you can test by cutting a solid 'shim' to shape and slide it under the balls for the time being? Something like an overhead transparency sheet (cut to fit) works well just to test. Just lift the PF and slide it in from the left side. No need to disassemble at all.Shape.jpg This is the basic shape......

Post the result. It worked for me on a couple of machines.

You can get balls that don't magnetise as well. Probably a good idea regardless? Carbon steel type?

Also, make sure you have the correct number of balls installed. No offense, but it can happen!!

Failing that, there may be an issue with any switch that causes a ball to be kicked back into play? I would check the optos for the goal area. As a start, go into diagnostics and see if any switch is stuck on?