Why Does My Jeep Hate Vaccum?

[N2URHED]

I have an 89 Jeep Grand Wagoneer that has been giving me fits lately. It would stall for no reason at the most inoportune times. (It should be noted that the previous owner removed all EGR stuff and plugged up the vacuum sources). So after much reading I began to replace suspect parts. The short list is: Carb rebuilt, fuel filters changed, fuel pump replaced, coil replaced, pick-up coil and trigger wheel replaced, ignition control module replaced.
The last bit did the trick. With the new pick up coil and ICM I do not experience any stalls. This is good. But my question is this:
Why is it that when I set my timing according to the label and instructions at 12 + or - 2 at around 600 RPM with vacuum plugged it sputters and coughs and will not run right when I hook up the vacuum again? If I disconnect the vaccum advance it runs fine. It does not sputter, it does not ping, it does not backfire. It is a little hesitant when cold, but responds fine under load throughout the RPM range. I am perplexed. How is it advancing without vacuum?Doesn't it need vacuum? Why does it hate vacuum?
It had vacuum hooked up before and never complained.
I can't even guess why my Jeep is behaving in this fashion.
Any help would be greatly appreciated.

Thanks,

Jeffrey

[Lugnut]

Did you check to see if your vacuum advance on the distributor has a hole in the diaphragm?