Slant 6 Overheating

One guess is your thermostat was sticking shut, maybe erratically. Could be that when you saw good water flow, it wasn't stuck. My dad's 63 Valiant slant overheated on the freeway when I was a kid. A road worker looked at it, saw no flow, pulled the T-stat and found it stuck shut, said we didn't need one in FL (not really true) and my dad kept driving it w/o T-stat for 6 yrs. I was never able to see water flow in my 69 slant or 65 BB since the rad has a baffle around the inlet.

A bad rad cap gasket can slowly lose coolant as you drive, but probably not in 10 min. A lot of shops make money by routinely using a cap tester and replacing caps that are OK (good gasket, spring slightly weak).