Vaccuum testing

My 92 XJ 4.0 had been cutting out under sharp throttle and at high rpms, and I've traced the problem down to a vaccuum leak, but cannot seem to find it. I can't find any specs either for what the vaccuum pressure is supposed to be. I've replaced the throttle body and intake manifold gaskets, and replaced all soft vaccuum lines. I'm consistantly getting a reading of 14-15 psi of vaccuum, and it dips to 20 psi when the throttle is let off and immediately regulates back to 15. When I hit the throttle sharply it goes to 1-2 psi of positive pressure, then regulates back down to 15 psi vaccuum. Is a crack in the intake manifold likely at all? I've been doubting it, but it only seems to leak under large pressure differentials, and I can't hear any leaks over the engine. Any advice or numbers I should be looking for would be appreciated.

I haven't tested my Jeep to see how it reacts to sharp throttle hits, but this thread will show you some numbers I recorded a while back...it also has an article about troubleshooting with a vacuum gauge...hope it helps...
http://www.naxja.org/forum/showthread.php?t=945353&highlight=vacumn

Amazing what's on youtube. http://www.youtube.com/watch?v=iPwkmN_pPMA

To check for a vacuum leak you can't otherwise find, use a can of carburettor cleaner. From a cold start, spray a fog around everywhere a leak is suspected (include any vacuum fittings, and the intake-to-cylinder head mating surface.)

If you note the problem when cold, but it lessens or goes away entirely when hot, check torque on your manifold screws (particularly the five around back - very back, two top, and two bottm.) I check them annually as a matter of course, and invariably find at least two of these loose. A 9/16" "flex" socket (shorty socket with inbuilt U-joint) is terribly helpful here, and requires no torque adaptation.

When you have a side-draught head with an aluminum intake and a steel exhaust, loosening manifold screws is very common. I could explain why, but I've done so here in the last month and I'd really rather not repeat myself...

Thanks, I'll try and use some carb cleaner if it doesn't go away. A friend suggested I check the PCV and sure enough the check ball isn't making any noise. I took the vacuum line that exhausts from the PCV into the intake and put a T fitting connecting it to the vac. line going to the MAP sensor. I spiked the idle repeatedly and it doesn't seem to be stumbling, I'll take it for a drive tomorrow and see if that was the problem. I torqued all the manifold bolts in the specified order with my torque wrench when I installed the new gasket. If anything, the problem gets worse as it gets hot since the engine isn't pulling enough air to match the fuel it's giving the engine.