A. Do the ground mods, very important.
B. Note the jumper to 120V will LOWER unregulated voltage, which includes solenoids. From pinrepair.com that you saw:
"This change should always be made to jumper the game for 120 volts.This keeps the unregulated voltage (such as the General Illumination) lower, for increased bulb life."

Do the ground mods first. It's not really easy to measure "power", because the solenoids depend on the on current delivery. The fact that the game runs for a bit fine, and then changes is indicative of a temperature related problem. It could be as simple as ground return(S)...or it still could be something else, even a bad transformer. Just don't be too hasty and start changing parts randomly.

Either method will work. I used the method that maintains the original A18/A12 ground connectors.

Don't replace anything yet. Coils are rarely the problem. Your EOS switch in the upper right looks filthy from what I can see. It probably needs to be cleaned. If that is pitted or oxidized you will have these kinds of problems. Same goes for the flipper cabinet switch contacts.

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You'll need a tungsten point file to clean up EOS and cab switches, even then requires a bit of time and elbow grease. A business card is only for cleaning very light dirt from low voltage plated switches