Thanks for your response.
I found the source of the wandering spur. It is a subtle temperature dependent interaction between a voltage regulator and an 3rd overtone voltage variable crystal oscillator in a phase-locked 48MHz reference. Apparently, slight shifts in supply voltage would intermittently induce the excitation of a spurious mode of oscillation that would interfere with the desired mode. Since the frequency multiplication is so high, the phenomenon was not visible on the reference signal spectrum. The problem was only confirmed by using a known clean reference for the PLL. By changing a couple of capacitors in the crystal oscillator feedback network, the problem went away.
At the end of that day, the ADF4113HV was working perfectly. I only initially suspected the digital dividers because of having seen the exact same effect with the fractional-N PLLs. Since this is an integer N PLL, the frequency dividers cannot have this behavior.
Again, thanks!
Bill