Unfortunately, it looks like I've suffered the A3 power amplifier failure that many others have seen. In my case it was a slow death, the maximum output power slowly crept downward over the course of a few months and now the unit can no longer phase lock reliably. ID+ to ID- measures 900mV, indicating an amplifier current far higher than the nominal 200-400mA (200-400mV across ID+ and ID-).
Fortunately(?) it looks like at least at some point a replacement amplifier was available, see service note 8753C-03:
I would love to get my hands on one of those parts kits, however the only place I can find reference to them is in that note. Does anybody have a lead on one, or am I going to have to design a 300 kHz-3 GHz 25 dBm amp? Thanks.