My guess is one of four things, divide and conquer:

1) playfield wiring issue, intermittent short to ground
2) drive circuitry going out (pre-drive or drive transistor)
3) logic circuitry that feeds the drive circuitry flaking out (changing logic state when it shouldn't, etc.)
4) PIA is flaking out

Can eliminate #1 by unplugging control line connector to coil from board

If issue remains, put a logic probe on the base of the pre-drive transistor and see if there is a state change (low to high) when the diverter fires. If not, replace both the pre-drive and drive transistors.

If there is a logic state change at the base of the pre-drive transistor when the coil fires, put the logic probe on the PIA output pin than controls that coil channel. If there is a state change there, replace PIA. If not, replace logic chips between PIA and transistors. Also check resistor array.

Last bet is a factory reset to exorcise the demon.