Instant and One Time Crankcase Overload????

[whunderboy]

I have an odd question for you guys to ponder but first let me lay out the car.

I have a 1998 Nissan Altima GXE Engine: KA24DE Unmodified; Timing chains & hardware replaced at 130K (read: overhauled ;P) This was 8000 miles ago. Transmission: 5 speed Manual Gear lube replaced with Timing chain.

One day, I was driving along, minding my own business. I pulled up to a stop light and noticed there was a slight lope, and the RPMs dropped from about 900 to 800 or so. After about 10 blocks the car started REALLY running rough. It was also smoking blue. Not alot but enough to set off alarms in my head. I pulled over and turned it off. looking under the hood I saw oil seeping out of a crack in the exhaust manifold (It is a crack I have known about but haven't taken care of yet.). I let the car sit for about 15 or 20 min and then (at my wife's urging "It's hot in here!") I turned the car back on to get the air going for a few seconds. It ran like new ! No new oil, no smoke, no stuttering at any RPM... I have changed the PCV valve (the old PCV valve was fine) and checked the compression (yes I used the "proper procedure" to test compression) (178,176,177,181 PSI). I am fairly confident that this won't happen again but was wondering.... What the HEEEEL happened? What would cause the crankcase to SUDDENLY pressurize enough to pop the oil pan seal AND force oil into the combustion chamber(s)?