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AD7927 issues when powered by 5V rather than 3.3V

Hello,  I am utilizing the AD7927 ADC that is found populating the PHYTEC phyCORE-BF537 board.  It appears according to the schematic that their engineers have wired it up correctly.

I am connecting to the AD7927 through the SPI port on the Blackfin ADSP-BF537.  I am using the same code for both when I power the ADC with 5V and when I power it with 3.3V, only adjusting the VREF or 2*VREF bit accordingly (though I have tried all four combinations.)

My issue is that when I power the ADC with 5V, it behaves strangely.  When I power the AD7927 with 3.3V, all is as expected.  I can select channels, and the channel data sent with each conversion agrees with which channel I was expecting.  Also, the measured voltages match the applied voltages.  But when I power the AD7927 with 5V, it will only work with Channel 0.  If I select other channels, the returned channel address bits always return 0, and it continues to read from Channel 0.  Also, the reported measurement bits do not correspond to the voltage applied, though at least they are consistent; if I take the same voltage reading over and over again, the bits will not vary.

I have measured the pins, and they all make sense (5V or 3.3V on the two AVDD, Vdrive = 3.3V, VREF = 2.5V).

Any help with getting the AD7927 to work with a 5V supply will be very appreciated!  Thank you.