Inconsistent A/F at idle/neutral

[Shark Racer]

While setting my idle and trying to set the APT, I noticed some irregularities on my AF gauge.

I started out with a relatively rich idle (~11:1) and dialed it in a bit. As it got into the 13s, the idle started to vary pretty widely, faster than I could read but bouncing between 14-16.

The story is similar when the motor is held at a constant 2100 RPM in neutral.

Any idea what could cause this? I'd expect the idle to be able to stay rather consistent. Mixture screws are on the order of 3 turns out and I'm not running much by way of throttle opening - they're nearly shut and idle is ~650RPM. (216 / 216 110LSA cam in a SBC 350).

[77cruiser]

Sounds like a misfire. If a cyl. misses it will show lean.

[Shark Racer]

What's strange is if it's a bit too lean or a bit too rich I don't see the lean spikes that I associate with a misfire.

Could a poorly matched power piston spring cause this?