If you have low coil resistance on some coil in the game it can stress the transformer enough to cause the hold/lock coil to de-energize Long enough for the self hold switch to open, making the game go dark. Or some sort of wacky short on coil power could also do this

I once saw this happen on a game which was missing one of the chime coil plungers. Without the plunger, the coil would draw excessive current momentarily, which pulled down the transformer enough to allow the hold relay to drop out. Installing the plunger fixed the problem.

Not the hold/lock coil, but I had this very thing happen recently with my Atlantis.

I installed two of PBR's yellow dot bumper coils in the game. Immediately, the P relay, which holds on to light the standup targets on the right side of the playfield, would drop out as soon as a bumper coil fired. The slings or flippers didn't cause it, just a bumper hit. I was sure I didn't do anything else during the installation, but nothing I checked would fix it. Finally, I just put the old coils back in and the problem went away.

Those yellow dots measured 1.5 ohms. The original coils measured 2.7 ohms. When you figure it, those coils caused the amps pulled by them to nearly double, and I guess that was enough to cause that coil on the P relay to let go.

I have since done the mod where you take a few turns off a bumper coil for a different game, but I measured the resistance along the way and kept it to about 2.2 ohms. Enough to give the game some extra kick, but not enough to possibly have issues like I had with Atlantis.

So, now I have 3 yellow dot bumper coils I don't want. Anyone want them? I'll send them to you for the cost and free shipping.