I would check for any cold solder joints under the MPU connector header J4.
Then, also check for any broken or corroded 0.100 pin(s) inside that connector housing.
Why? Because:
- Solenoids are 'commanded' from MPU through J4 (then through the Solenoid Driver then drive transistors there);
- If there was any corrosion at any point on this MPU or earlier/original MPU, corrosion from the battery often migrates downwards
including inside connectors J4 (bot left) and J3 (bot right).
Note: I also have seen intermittent diodes on the Solenoid Driver board affecting one coil at a time (not two together like your case).
Good luck!
- Sylvain.