DAC5688 spurs problem

Hi, I am using a DAC5688 with a Pentek 5973 VPX carrier card, and having some problems with -60dBc spurs +/- 40MHz from my test CW signal.  I am feeding a CW signal using interleaved data mode to the DAC5688 at 200MSps, 4x interpolation (DAC clock at 800MHz).  If the NCO is enabled and my Fdata (freq of input signal) is above ~50 MHz, the output looks clean.  But if Fdata is less than ~50 MHz, the 40MHz spurs occur.  I believe they also occur if NCO is disabled, at any Fdata.  I have tried setting the FMIX gain to -3dB, and QMC gain to 0, but still spurs.  Any ideas where to start troubleshooting?  Thanks!