I think no one mentioned that a broken U14 (U23 on WPC-S and WPC-95) can take out U20 after a while. I think that U20 normally breaks within a couple of minutes if U14 is broken, and it sounds like you can play longer than so, but it could be worth a try, changing U14.
One way to check that there isn’t any short on the playfield is to disassemble the U20 chip, then start the game, and measure in the socket that it isn't higher than +12 VDC on any of the pins 11 to 18. If it’s higher voltage than that on any of those 8 positions, then there’s probably some type of short on the playfield. '
If the short is intermittent, you probably won't see a higher voltage in this test, but it also could be worth a try.