So I was helping a friend with his Haunted House (HH) pin. The bottom playfield stopped kicking the ball back onto the main playfield, and the two right score displays and the center credit/ball display would distort on a cyclic basis – they would show all segments in a flash-flash-pause, flash-flash-pause pattern, and this happened when the game was in attract mode, test mode, or being played. 

The playfield problem was from the three connectors for the bottom playfield not being routed correctly and he accidentally unplugged them when lifting the main playfield – two of the three of them were disconnected. We routed them through the little slot at the back of the bottom playfield, connected everything up and it all started working again.
I then noticed that the displays would freak out when the “lightning” effect occurred on the right side of the backglass, which coincided with that flash-flash-pause pattern.  The lightning is handled by a standalone board that just accepts power and operates autonomously. Thinking of the known problems with grounding and Gottlieb pins, I started wiggling and reseating connectors. The problem seemed to change in severity, but did not go away entirely.

Disconnecting the lightning board resolved the problem, at the cost of no lightning. My friend mentioned that he has had (or heard that) dodgy sockets cause strange, seemingly unrelated problems. As we started pulling the old incandescent bulbs from the “lightning” the problem went away! He ended up replacing the bulbs with LED ones and everything worked great.

So if you have an odd problem that does not make sense, check the nearby bulbs and sockets!