Tried again today with a mag mount antenna. I noticed 2 odd things:
1. RSSI is not accurate at stronger signal levels. Almost every station I tried showed rock-steady -40, -45 -50, or -55dBm. Readings below that fluctuated in 1dB increments as expected.
2. The signal level is not correlated to channel dropout. 101.5 was dropping every 10-30 seconds, while 92.7 never dropped once. Both showed -45 or -50dBm. Noise level did not change during channel dropout.