Check the software revision. Some IJ's do have bugs when Eternal life starts, and once I have had a crashing IJ when Billion was supposed to be awarded.
Perhaps something there.

That does sound like battery corrosion on the MPU. But the problem which is mode-related does not.
No problem double checking that though.

The volume / menu switches are direct switches, which are not in the matrix. They are connected to J205 at the MPU.
But if you swapped the PCB's it is certainly not there, but in the wiring / switches.

Back to the IJ problem which slamtilts after completing all modes:

@OP:
- Can you reproduce the problem?

- Can you check what the game does at that time, and when it seems to happen?
Perhaps some flashers flashing (eternal life insert?) or something the game does at that moment.
Or when all lamps flash at the same time showing the ball needs to be shot in the modestart?

- What happens to the voltage at the CPU when all lamps are on? Does the reset also occur when in "all lamps" test?

- Is it really a slam (does it have the same effects (display / sound / etc) comparing to a manual activated slam?
Or is it a reset of the machine (caused by low voltage by flashing all lamps)?

I'm just throwing in some ideas here, hoping someone else may be triggered for a good idea....
Very strange that it only happens at that time. Does sound like software, but then it also would be a known problem. Then all L7's would be experiencing the same problem.

I think first thing is trying to reproduce the problem, try to figure out what exactly happens at the time the slam (or reset?) occurs.