First step is to check the vacuum pump. Attach a vac. gauge to one of the line running off the main vacuum source line that goes from the pump to the brake booster. Plug the other ones. Start the engine and take a reading. It should be in the green and steady. If it is low or jumpy, rebuild or replace your pump after testing your brake booster.

If your vacuum source is good, then it has to be one of your components leaking off the vacuum.

Connect a mityvac type vacuum hand pump to the fuel shutoff valve on the back of your IP (a round metal bodied valve with one brown vacuum line entering it at a rubber connection). With the engine running, pump the hand pump a couple times. If the engine quits immediately and holds the vacuum, then you know that at least your fuel shutoff valve is good. If it does not shut off after a few pumps or if the vacuum leaks out, then you need to replace the valve.

If your valve is good, try plugging off each of the lines running through the firewall (the end towards the vacuum pump) in turn and seeing if your vacuum shutoff problem stops. That way you can isolate it to one system (AC, door locks, etc). Door locks is a common culprit.

Good luck

__________________
"I would rather be ashes than dust!
I would rather that my spark should burn out in a brillaint blaze
than it should be stifled by dryrot.
I would rather be a superb meteor,
every atom of me in magnificent glow,
than a sleepy and permanent planet.
The proper function of man is to live, not to exist.
I shall not waste my days in trying to prolong them.
I shall use my time."

-Jack London 1876-1916

1983 300CDT (running WVO since 12/05)
1981 300SD (parts car)