2004 Honda Civic Coolant Leak

Since mid-winter, the overflow tank on my 2004 Civic EX (original owner) is full to the top rather than maintaining the 'normal' level. Small amounts of coolant is being pushed out the top of the tank. The coolant level in the radiator drops and must be replenished by adding about a cup every 3-400 miles. The engine has continued to operate at normal temperatures and starts and runs fine. The system maintains some degree of pressure even after sitting over night. Its as if the coolant, rather than the 'air bubble' expands into the tank and air is drawn back into the radiator? I haven't noticed any white smoke (or anything abnormal) in the exhaust. The coolant, oil fill cap, oil dipstick and transmission dipstick all show no contamination or turbulence. Although I doubted it would help, I changed the radiator cap. I would change the thermostat but I doubt it would help. Am I looking at a head gasket leak? Something else? Thanks!