Also be sure to add fuses to the 2 backbox-mounted bridge rectifiers, as detailed in vid's guide fatpanda posted above. It's a potential fire hazard without the fuses! Here's the specific section of the guide I'm referring to: https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6/page/2#post-945742

Do the coils fire in coil test?

Look for a broken wire at the non-working coils. It's also possible a wire could have broken off another coil upstream that is "daisy-chained" to the ones that stopped working.

Next time the game starts tilting, access the switch test in diagnostics, remove the ball and make sure all drop targets are up. See which (if any) switches come up on the display, it will show a number for each switch that it currently detects closed. I'm guessing you may just have a tilt switch that needs adjustment, or you may have to adjust the tilt bob...you could even remove the tilt bob temporarily to see if that fixes it. The switch test will hopefully tell you which switch to focus on.