all of my cooling system is new, EXCEPT the radiator, following an engine swap. figured with everything apart i'd give it new hoses, tank and thermostat. thanks for the tip on the radiator. i really don't want to replace it (i feel like all i do is work on this car!) but it's definitely better than a head gasket.

i read in the FAQs that a GREY expansion tank cap is susceptible to cracking in the threads which can lead to coolant oozing out when the engine is hot. if you have a grey cap, consider replacing it with a GREEN one and maybe that will help with your slow loss.