Noisy AD9951 output

I have built nearly 300 AD9951 boards that have been very successful. However, when I had

new boards made, I ordered (40) 9951's and had a reputable SMT house install them. Building two boards

showed an extremely noisy signal (clock noise). With a low-voltage ohm-meter I checked an old

(bare- only 9951's and 1.8 volt regs) board against a new one, just going through the traces

(no measurable resistance in the traces or grounds), the solder joints (small but negligible

resistance) and then the chip to ground. Here is what I found:

         Clock input of 9951 to ground:  old board, 500K ohms, new board: 520K ohms

        1.8 volt regulator output (through 9951) to ground: old board, 48.5K ohms, new board: 50K ohms, etc.

The resistance through the chip is different, indicating a possible change in the chip mask or something

of that sort. The new chips exhibit a consistently higher resistance. The marking on the chip is as follows:



                                                       $  208


Is there a way of finding out if these chips were changed in any way? I imagine you have field data.

I need to resolve this issue right away and I would appreciate any guidance you can give. There seems

to be a significant change in this batch of AD9951s. Several old and new boards give exactly the same

results. Also, I had A.C. re-flow the soldering, but I measured the same results.

                                                Jim H.