Saint-hyacinthe, QC
Could very well be the ram. U8 is a 2064 or 6264 SRAM. Your game would benefit using a NVRAM, removing the need for batteries and possibly fixing this issue. With all the tests you've done, there aren't many possibilities left. RAM / CPU / ASIC would be my guess, in order. RAM is always prone to failures over time and if it's the original part, a long time has passed since the day is was manufactured. A bad CPU would mostlikely lock / reset at other times as well. I'm also guessing a bad ASIC would give more issues than a precise one like this. I would definitely change the ram to start with.
https://www.marcospecialties.com/pinball-parts/RAMTRON
https://arcarc.xmission.com/Pinball/PDF%20Pinball%20Misc/Williams%20WPC%20Schematic%20manual.pdf
San Martin, CA
