Thanks dusty - that gives some peace of mind.
I read online somewhere yesterday that narrow band o2s are kind of binary and will typically flip from low to high voltage when the af ratio becomes rich or lean. So some fluctuation is probably normal there too when running on o2s.
I tried a stethoscope and shooting some starter fluid around to look for vacuum amd exhaust leaks but nothing conclusive.
It does seem like a vacuum leak is the most likely culprit based on the symptoms
I noticed the vacuum hose that comes off of the t at the intake manifolds and runs to the pump under the dash is kinked a bit, but not leaking - I wonder if this could cause running
One way or another looks like I've gotta start taking stuff apart to get access to those hoses