First try the external reference again, to be sure whether the internal one is OK or not. In my experience, the (Ovenaire brand as I recall) OCXOs used in the 8566s have been quite unreliable (ovens crap out), unlike the good old HP-made ones. It may be the application environment rather than the part being inadequate, however. As I have mentioned before, I don't like that the OCXO sits right in the high-flow center air plenum, so its oven has to work very hard to keep it hot while running.
Ed
