I have had this. Try to figure out how many balls are in the trough to trigger this. It usually occurs when more than one ball is on the playfield AND you hit a certain switch. I would see if its simple and just on the row 1 where the slam and the trough optos are but in my case it was more complex as the optos do weird things.
If you figure out the number of balls on the playfield when this happens you can start multiball put the remaining balls in the trough and trigger switches until you get the slam tilt. Then go from there.
In my case, I had a smashed diode where the wires touched without needing the diode. that was causing the switch matrix problem when certain combination of optos in the trough were on/off and the CPU interpreted it as a slam tilt when I hit a switch.