Honda Civic Heating Problem

My 2005 Honda Civic has not been heating the car properly. At low RPM iI get no heat at all and at high RPM I get “OK” heat. Dealer drained radiator, which was low, and put new and put new coolant in and checked thermostat. There was air in the cooling system but that was supposedly taken care of. Dealer said that if that does not take care of the problem then it is probably a bad head gasket. After running the car for 3 weeks I lost coolant again and dealer says there are no external leaks so it is gasket. Advice?

How many miles on it? Ever overheat the car? (boil-over). Do you have oil in your coolant or coolant in your oil? Check your engine oil and post back what it looks like. Rocketman