Many thanks to Dennis, Flying Camera, Ian, Doug and Jeffrey!
The mottling is more of a touch of gray that is not quite uniform in the highlights (I apologize for any confusion with the term). I just wanted to make sure I wasn't missing something with the metal mixture when using dichromate in the developer.
It appears that Pt/Pd is much more sensitive to stray UV then Pd with NaPt for contrast control. Tonight I will try coating a sheet of cot320 in very dim room and will be militant about keeping the sensitized paper in the dark until dry. In the past with Pd (NaPt), I was able to coat inside during daylight hours - I tried that once with Pt/Pd and got a dark gray mess.
Usually not a problem in NC, but out of habit I typically steam the paper over a kettle before exposure. Starting to think this may also be contributing to the problem and some of the variability. Will try without steaming.
I am biochemistry professor by day - so fairly confident my solutions are OK (although I have been known to make boneheaded mistakes - which of course I blame on my students).
I will make up fresh FO (cheap enough to mix on demand). Given that Pt/Pd appears to be more sensitive to stray UV light than straight Pd, it may also be more sensitive to the age of the reagents .
Almost always done between 20C and 37C
Many thanks for all the help - despite the cost of (Pt) and my problems with the highlights - the overall tone of the prints are fantastic. My Pd prints were always a touch too warm for my taste and these are perfect. Once I dial in the 1:1 ratio I will see how low I can I dilute the Pt for the same effect.