I sure hope you are keeping an eye on "signs of oil in the coolant" and "coolant in the oil".
Did you have this problem prior to replacing all the components?
How many "starts" or trips does it take for it to lose the half gallon?
What is the overflow doing? The pressure should be released through the overflow, not blowing at the cap. Try putting a catch can under the overflow hose so you can see what might be lost through that route.
I think a pressure test "cold" and "hot...warm" would be in order. Although, you mentioned it doesn't overheat, does the temp gauage flucuate or stay steady on whatever the normal running temp reading is?
It seems to me a "spotting on the radiator" wouldn't equate to a half gallon + in a week. Good luck.
