Since it is RWD, it will be a little easier to troubleshoot.
I'd strongly suggest a Mighty Vac. Put it on the radiator neck and pull a vacuum (doesn't need to be much) to see what the rate of decay is (kind of a leakdown test for cooling.)
If you see any movement, you might be degassing a bit, try it again.
If it KEEPS happening, then you would pressurize slightly and swab some soapy water around the connections to see if there is bubbling.
Also, I'd invest in a test of the coolant to see if there are any combustion byproducts dissolved that would indicate a head gasket leak (not uncommon in the Ford V6s)
Good luck, hope you get it sorted!