Thank you for your response. It looks like that is the problem. I thought I had tied the VCM output from the 9255 to the VCOM input on the 5562 using JP8 to account for that, but I neglected to connect jumper JP1 to complete the path. Having now done that, the output level jumps I was seeing go away for the most part (as long as limit my signal levels). As you pointed out, however, the common mode voltages are not really compatible, so I do still have some issues if my signal levels get too large. I will keep working to find an optimal VCOM for the 5562 to give me the most range, but for now this is a big improvement!
Thanks again,
Greg