So, on your game, the CPU uses one transistor to both activate a coil, and a flash lamp.
This is why you use the coil test for the flash lamps.
Either the coil is powered through the A/C relay, or the flashlamp is powered through the A/C relay.
Your manual is here:
On pdf page 2, you have a coil/flashlamps list.
You'll notice that Outhole Kicker and 'Top Ctr Ln Insert Teeth Flasher' are both connected to Q33. The PIA controls Q33, and whether the A/C relay is engaged or not engaged is what determines if a coil pulls in or the flasher flashes. On the same chart it says 1b/1p. This means that there is one flash bulb on the 'b'ackglass, one flasher on the 'p'layfield. So multiple flashers should light.
While it could be a PIA problem, I would suspect that you'd have multiple coils pulling in the same way your multiple flashers are activating.
If they don't flash at all, overwhelmingly it is likely that the bulb is burned out, or the 906 bulb isn't making good connection in it's socket. (shrugs) could be broken wire. It is also common for the interconnect board in the backbox to have broken solder connections. The interconnect board also has big cement resistors that can be open.
If multiple inappropriate flashers are activating, someone may have rewired your game (always suspect prior work!), or something stranger may be happening. In either case you'll need to go into coil test, and freeze it on the suspect flasher, and see exactly what bulbs are coming on when.
Let us know what you find!