Quoted from NM:UPDATE:
Wow, what a 180-degree turn:
Markmon, your instinct was 100% right--I just tried the game after it sat all night and it's back to resetting every time both flips are pressed simultaneously (with Ball 1 in the shooter lane and 0 points on the DMD)!
I'll continue plotting through the guide later to pinpoint the cause, but what's the usual culprit in a case like this (when a game only resets when cold & not after it's warmed up)?
Thanks.
Maybe the Thermister?