LMX2595: Single Frequency Problem

Tool/software:

All

I am having trouble with an LMZ2595 synthesizer. It will produce almost every frequency I want (simple single frequencies, no modulation or chirps). But on some PCBs the part will not produce 2.5 GHz. There is a small band of frequencies a few hundred MHz) where this is a problem. it starts with some spurs, then goes to total noise on the spectrum analyzer.

I have looked at the charge pump output, both before and after the filter, to see if there is anything odd; but the signal doesn't look different between a good frequency and a bad one. I have looked at AC and DC coupled to see if either showed any oddities between good and bad operation.

Any ideas would be helpful.

Ian