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ADP2120 operating in a discontinuous mode when synchronized.

I have an ADP2120 circuit that is operating it what looks like a discontinuous mode, but is synchronized to an input clock.

The circuit is a 5V to 3.3 V circuit using the component values I got from ADIsimPower. The current draw on the 3.3V varies vs time. The switch waveform has an odd ring that moves around. The switch turns on (0V out) for ~200 ns, then softly releases and slews up to ~4V, then drops to 3.5V before sharply turning off (5V). The switch waveform looks a little like the DCM (discontinuous conduction mode) in the data sheet, but with only a single broad ring. The circuit is synchronized to to a 1.2MHz clock (3.3V) so it shouldn't try to go to the PFM mode.

I haven't found any info about the DCM in the ADI web site yet. I have another ADP2120 operating at 5V to 1.4V that behaves nearly ideally. Any ideas or suggestions as to what this is & how to make it operate in the continuous conduction mode?