Foaming/cavitating coolant in my 34

[Chris Marshall]

Questions for the early pump in head flathead gurus. I have a 34 roadster with a later (35 or 36) flathead. I've driven it a decent amount this summer, but have lately been frustrated with it pushing out all the coolant to the point it starts running hot. Here's some of my experiences this summer:

Drove it around town in 100+ degree weather (107 actual) with no issues. Ran good, got to 195 idling at stoplights. Never pushed out any water.

Driven it on a couple of different occasions on 120 mile plus trips at 55 mph or so. Ambient temps near 80 or less. No issues what so ever.

Driven it on extended drives with ambient above 90 and the coolant will foam (pumps cavitating?) at speeds above 50mph. Temp stays at 180 (it has a mechanical gauge added) until the coolant level starts down then slowly rise as more coolant is forced out. Once this process starts even slowing to 45mph doesn't slow down the coolant loss. Cars behind me have a shower of coolant!! It can loose as much as a gallon of coolant in 20 miles under these conditions. When the cap is opened, the coolant is foamy.

This is frustrating. I may be drawing the wrong conclusions, but it seems to me if this is driven by a compression (head gasket or crack) leak ambient temp would not come into play.

By the foamy appearance of the coolant and the higher rpms necessary to start the process it seems to be cavitation at the water pumps.

This is a stock, non pressurized system.

I'm open to ideas/experience/etc. This is a great car to drive, but at this point I'm limited to driving in cooler temps. I forgot to add it has a 4.11 rear.

TIA