Followup/Status Report:
I replaced the ribbon cable between the CPU and the Powerdriver board and that resolved the issue of no continuity between the Power Driver Board and the CPU. All switches are registering in Switch test mode. The game boots up and plays fine, but when I press both flipper buttons, (all four flipper buttons activated) I am still getting the slam tilt and the game reboots. I tested voltages with the game in diagnostic test mode and got the following:

TP1 (12v unregulated) -- 14.89v
TP2 (+5v) -- 4.98v on the Power Driver Board and 4.96 on the CPU
TP3 (12v regulated) -- 11.8 on Power driver board and 11.75 on CPU
TP7 (20v) 21.9v
TP8 (18v) -- 18.3v

I started a game and measured the 5 volt and 12 regulated voltages with one flipper button activated. The 5volt stayed at 4.89 on the CPU but the 12v regulated immediately dropped down to between 6.56 - 10 volt which makes me think I have a problem with the 12 regulated Power supply. Comments or ideas?

Thanks Zaza's! That is it 12vreg dropping way down when one or Both sets of flippers energized. What parts on the power board would be suspect? Could CPU be suspect also
?

Here are results with and without pressing both flipper buttons in T1 Diagnostic test:
TP 8 -- 18.18/17.92
C3 -- 16.8/16.42
F115: 11.76/6.8v.

So the voltage drop is occurring after C2. So it looks like C2 is bad not BR1? Could Q2 be bad?

I will pull the board, inspect, and replace C2 and post after I get board back in game.

I replaced C2 and also C4 while I was at it. C2 had failed and started leaking electrolyte. Cleaned board and inspected traces, they were good. I used a fiberglass fine point pin to first clean the old solder joints on the old C2 pins and used lots of liquid flux. I added a little fresh solder, then I I used my new Hakko 472 Desoldering station gun to desolder the old cap and removed it. Once electrolyte leaks, it will contaminate the solder and requires special care to desolder the bad component. Just need to take your time and be careful. I next installed the board and played several test games. All is well, activated both flippers during game play at same time and no slam tilts or reboots. thanks everyone for helping me diagnose and resolve my problem, especially to Zaza for the great schematics and nailing the problem and providing the steps to trace the problem. It is satisfying to troubleshoot problem and repair in a systematic, way, not just shot gun things.

lessons learned:
1) The Cpu to Powerdriver Board ribbon Cable can go bad and not provide good connectivity: When in doubt replace them.
2) slam tilt and reboot and other strange behavior can be caused by failed or failing 12v regulated circuit, always check/replace C2. I think these are prone to failing and leaking in these older games, especially if the cap is original. Perhaps because they sit close th the voltage regulator/heat sink.
3) don't be afraid to ask for help here on Pinside. There are lots of extremely talented folks here that are willing to help those who want to help themselves.