I had the scanner read sub-500 mV for the O2's in closed loop repeatedly. I had both a knock count and a knock reatard reading several times.

 

The O2's should go up and down from 0xx to 9xx and as long and you get something like this you are fine. 2xx 7xx 8xx 4xx 6xx 3xx 7xx 2xx and so on. You don't want them to stay in the same area and bouncing around means they are working correctly.

 

I had three or four readings in a row that were all sub-25 while the engine was stumbling. There is definitely something funky going on.

 

I checked my old datalogs and had four reading below 75 on one side of the O2's. I ended up having a bad alternator and once I replaced it all my issues went away. I only had issues at high prms when it would stumble. Probably not your problem, but it was mine.

 

My Z had similar symptoms. Would run fine then out of no where would start to bog. Even with pedal floored, car would do nothing, low idle. Turned out to be one of the plugs arcing. You mentioned you checked wires, did you recheck plugs. After I checked plugs, Z was back to purring. Good luck buddy.

 

Was it just not in all the way and arcing on the outside of the block?

 

If anyone's interested, I figured it out. My cat was really clogged, maxing out my pressure gauge at 10 psi. This is most likely the cause of the oil leak too, forcing oil out, so that may not even need a repair. I believe the cat got clogged before I replaced the ignition coil wire/plugs and wires, since the engine should have been running very rich.

My only problem now is disconnecting the y-pipe from the manifold. The studs are pretty bad, and I snapped one last night. So now I want to replace the whole set. Since I'm not worried about hurting them, anyone know how to get the studs out?