First thing you need to do is clean the board contacts and re-pin the connectors
Then you should do all the ground mods
Once you've done that you can start to work out any other problems

100% agree.

Until connectors are re pinned you will be chasing ghosts. I have restored a few system 1 and 80 games and in most cases the majority of problems resolve after re pinning and then a lot of what's left with ground upgrades.

Having said that, the best way to fix these games is to re pin and stick in a Pascal all in 1 board for modern type reliability.