Over the course of a weeks driving does it use much coolant?
Not the same car, but my Commodore had similar issues after I replaced the radiator and thermostat, new fluid etc.
I swore I'd done the head gasket, as after a few days there was little bits of oily crap on the rubber of the radiator cap. Then I checked the overflow, and it was in there too.
It was using around 100ml of coolant a day.
I found a heater hose that was a bit loose, and then dumped the coolant and replaced it. Its been fine ever since! The only thing I could think of was either the new radiator had a bit of crap in it, and the fresh coolant might have dislodged some buildup from the engine somewhere.
Check the overflow bottle to see if its got the same stuff in it