I just discovered this thread and need to add my 2 bit experience.
We have a 1937 Packard Super Eight. Several years ago it started foaming when I would drive 45mph for 5 minutes.
It NEVER overheated.
I tried all suggestions of
a, pouring some anti-foaming chemical into the radiator
b, buy a kit that can detect if combustion gasses are getting into the cooling system.
c, change coolant solutions.
I tried the first two with no success.
Finally, I decided to just go back to basics.
I brought the car into my shop, removed the water jacket and cleaned all the rust off the cylinders. ThenI removed the upper radiator hose and filed all the rust stalactites and stalagmites off the top fitting to the radiator, and the adapter to the head.
I put it all back together....and now I drive at any speed I want ...for as long as I want....with NO foaming.
Ihope this method will also help other collectors out there who are frustrated with foaming.