1996 Honda Civic Overheating

I have a 1996 honda civic lx. I am having problems with it overheating. I have replaced the thermostat about a year ago, flushed the coolant, replaced the radiator cap, bought a new coolant reserve tank. There is no sign of leaks to the naked eye. The fan seems to be running. When I turn on the engine, it revs up to 2 RPMs and sometimes will dip down to zero and jump right back up to 2 and then stay there. After idling for a little bit it starts to heat up and and coolant starts to leak from the radiator cap. Do you have any idea what the problem can be? I'm afraid it might be a blown head gasket or something more serious, is there anyway to check? Also my engine light has turned on. What is the cost of repair if taken to a mechanic? Thanks.