williams flash switch matrix problem

[Juventino]

i have a williams flash pinball and started having problems with some of the switches. using the switch matrix i found that any switch in column 4 will trigger every switch in that row. all wiring looks fine diodes aswell does this mean its on the boards? btw mpu and driver boards are near new.

[Homepin]

Diodes can "look fine" but be dodgy.

I would replace them first. This machine is 30 years old and these diodes have copped a flogging vibration wise.