Hi, thanks for the advice.
No, when I removed either of the two joysticks the problem is not there. The motherboard has never been recapped but visually it looks clean.
Thanks for that info. Did the graphic glitches persist with the joystick disconnected?
With the joystick plugged in, you could try manipulating the cable (particularly around the strain relief where it enters the stick) to see if there might be a broken/marginal wire within the cable. Kind of a long shot since the issue affects both joystick.
In my experience, bad joystick cables cause the stick to stop registering input rather than register spurious input. To register spurious input, the broken wire would need to short to ground. The cable on my Wico is shielded, but the shielding isn't grounded.
I would check voltages if you can. From the joystick port's perspective, the only difference I see between no stick and a stick connected with no input is that the signal lines (directional + fire) in the cable get pulled up to 5v. The cable will impose a small additional capacitance on these lines, perhaps enough to register an intermediate value if the power supply is flakey.
The joystick port supplies 5v on pin 7. This normally isn't used on a joystick, and in fact my Wico has no contact in that position. (If you look into the joystick connector with the row of 4 pins on the bottom, pin 7 is second from the right). Shorting that to ground through a bad cable could pull down the 5v rail and cause flakey behavior. Another long shot, but worth looking at your joystick cable to see if there is a contact there.
Robert