UPDATE:
I was looking more closely around at every unit, just to see if I could spot anything out of the ordinary, and eventually noticed a washer sitting atop the bottom set of leaf switches of the relay directly behind the big stepper in the bottom of the cabinet…
…marked RESET RELAY.
So that washer (which must’ve come off of some unit attached to the playfield) was shorting out the reset relay, causing the game to continuously (attempt to) reset. Fun!
But hey, at least I saw it and have it resolved now 
—————-
Hello, everyone! Just joined the club with an awesome Prospector! Got it home, has been playing fine, then randomly during a game, the two units pictured started running (5 clicks every time), non-stop, and that’s all the game will do since (even when turning it off and back on, this is all it does when it’s on). These two units just run, in unison, 1-2-3-4-5, 1-2-3-4-5, 1-2-3-4-5, continuously.
I’m not sure what it’s trying to do, but the unit attached to the playfield is the BALL COUNT UNIT. I don’t see anything that scores 5,000 that looks like it’s stuck on…not sure what might be going on here.
Am new to EMs, and would love any help/ideas you all might have! 
00EE5E21-00CF-4480-B0AD-24A15D631954 (resized).jpeg
68EECBA3-2BAA-45CA-A98A-F6F883370883 (resized).jpeg