Surge at idle

OK, it is an unusual engine - 1991 Vette (6-speed) with the original motor and original TPI manifold with a modified Dual Sync distributor. The engine is being controlled an a 0.5M 411 (OS 12212156, set to low resolution crank signal). It runs very well except one issue. At idle the RPM surges in sync with the HO2S' lean spikes; which are being driven by the STFT. This happens across the entire temp range from cold start to full operating temp. If I use the bi-directional controls to force Open Loop; the idle stabilizes. I've tried disabling B4108 but I still log STFT and see the HO2's cycling (and RPM surging). The VE appears to be reasonable because when driving, the STFT and LTFT numbers run -3% to -7%. Is there something else I need to do to turn off idle STFT?

Also: What determines which timing table (High Octane / Base Spark in Gear) the ECM uses? I understand idle. But, is it really throttle position? And the when does Optimal Timing come into play?