I have a '95 L36, and there is no spring on the PCV. But....it should not pull vacuum on the crankcase at idle. Sounds like you might be missing the o-ring on the underside of the PCV valve. If that doesn't stop the suck at idle, you may have another vacuum leak somewhere. That is not normal operation. That vacuum will pull your lip seals inward on the ends of the crankshaft and cause them to wear in an unplanned way. This can eventually result in a leaking rear crank (rear main) seal requiring engine or trans removal for repair, so it is not a trivial concern.
I had random stalling issues that were cured by the third (yes, third) replacement PCM. My symptoms were similar but not exactly the same as yours. Your description does kinda sound like it might be a fuel pressure issue.
Ummm, when you put in the new UIM, did you do the LIM gaskets too? If you didn't change the LIM gaskets, check out this monster thread. http://www.bonnevilleclub.com/forum/...ic.php?t=49487
The symptoms were pretty close to yours, and the problem was bad LIM gaskets.