Ya know it almost sounds like you have a air bubble in the system. Drill a 1/16 hole in the thermosat to let the air bleed out when you fill the system. Do you have a heater? Sometimes they will trap air also. A 7Lb cap would be OK also .Another thought some older radiators used a 1 inch deep neck most newer stuff is 3/4s inch deep, make sure you have the correct cap. Some of the older style radiators did not like too high of pressure ,Fords were famious for blowing out the tank seams in the 40s-50s if too much pressure was used.
