[Porchdog]

Car is an 81 911SC, Stock motor except for SSI's and MSD ignition.

I drove it to work this morning, about 25 minutes. After work, about 20 minutes into the drive it started running poorly. I have an AEM air/fuel gauge, as I passed 2000 RPMs the gauge would go to 16 and onto the peg. Upshift and it was better. Idle and low RPM's it actually ran pretty well and the mixture was around 14.7.

I reached my destination after 5 minutes or so of this and parked for at least a half hour.
When I restarted it immediately had the same problem, for about 5 minutes. At some point the problem went away (I was nursing the car on back roads so it was subtle).
Once I realized it was running properly again I tried to make it misbehave - large throttle openings at low RPM, running up the revs - it was mostly perfect "seat of the pants". Occasionally the AFG needle was bouncing around.

This car did this at least twice last fall. I unplugged the O2 sensor and it didn;t do it again until today.

Any ideas? I can't imagine what part of CIS could fail lean, then come back.