Had some time tonight, went ahead and
socketed/replaced IC6 (7408N) and it worked! I'll do more testing in the morning when I can make more noise without waking the rest of the house, but the Pop isn't locking on anymore, and when I started a game, I was able to trigger it normally.
The only sidenote is that I tested the voltages on IC6 Pin 1 & 3, and neither is 5V still. They are higher though, and tested at 3.99V and 4.43V respectively. Not sure if that means there might still be an issue, but for now it's working.
Any concerns with that?
Next Issue: Drop Targets.
They're resetting on ball drain, which is good. Previously, neither row was scoring or resetting during ball-in-play, so I took the center 3 target apart and cleaned it. The wiper contacts were dirty, as were the drop contact points on the boards. I cleaned them up, and reassembled. If I knock down the 2nd or 3rd target, no scoring occurs, and they stay down. Hitting the 1st drop target (furthest left) resets the entire group, even if the 2nd and 3rd drop are still up. After a short delay, the game awards some points. Thoughts as to why knocking down the first target always resets the row, regardless of the status of the other two targets? I'll try cleaning the back row of drops next.