OK, normally I just puzzle these out but this one has me stumped. So looking for some hints from some of you other board techs.
This 2518-17 mpu runs Leon's tests just fine. It has been modified to run 2732's at U2 and U6. I have tried a couple different sets of roms and confirmed the roms are good in another mpu. I have swapped and confirmed all the other chips in the other mpu board. I am getting the first flicker and 7 full flashes on the LED. THEN, instead of a half lit LED and a booted up game I get a quick half-lit flash and the LED goes dark. The 6800 is still chugging but nothing useful is happening. The voltages are fine including the +43V and the zero crossing signal is present at U10 pin 18. R134 has been replaced and has +5 on it.
Anybody seen this issue?

I should have mentioned that. I checked those pulses and they are indeed present at U11 pin 40.
The only oddity I have found in comparison to a running board is after that 7th flash the IRQA and IRQB pins measure 2.25V (although the logic probe still shows Hi/Lo pulses) whereas on a running mpu those lines measure 4.8V. I don't know if that is a symptom of a problem or just a consequence of a failed bootup.

Wait. Reread your post. Are you saying the 555 chip goes bad and can output "poor" pulses? (I haven't put the scope on the 555 yet. Maybe better do that....)

555 pulses look good.

Agreed. I tried several 5101s, 6810s, swapped the 6821s and swapped the 6800. Same result.
Frankly, I expected to find a solder bridge (or open trace) somewhere but I have looked and cannot find any.

Getting about 500Hz out of the 555.
The pulses on the IRQ line look about right as far as frequency but instead of being a square top wave they have a big slope to them. I think somewhere a couple lines are shorted.
Board came out of a Bally Mata Hari that has been sitting for many, many years. Slight battery damage - bead blasted and neutralized. I have replaced all the sockets. I know, way too much time but I do it for the challenge not the income and I have a "never die" attitude toward games. This board is nowhere near that bad.

Thanks for the additional responses gentlemen. I will investigate and report back.

It has! And my apologies.
I've been busy on a couple Hardtop projects and have not gotten back to this board.
But I will - it's on the bench. I have not forgotten.