Overheating Honda, what could be the problem?

Just a few days ago I noticed that steam was coming from the radiator cap so I replaced it. After replaced the car started overheating at high speeds but cooled back to almost normal temp after a while of stop and go traffic.
I checked the coolant levels and topped it off, I drove it this morning and yet again at high speeds the temp rose above normal, and cooled down again in stop and go traffic.
I turned on the heater when temp started to rise so I turned on the heater. Hot air blew for about a min or two then slowly only cool air would blow out. I turned it off and noticed that when left off for a while hot air would blow again but after left on a while only cool air would blow. When hot the rpms at pretty low and feels like if the car will turn off. Is this a thermostat problem? Is the rediator bad? Water pump? Don't have much experience with cars and could use th help

Sounds like lack of water in the radiator, an air lock or a water pump or belt problem

Percy has it right. Most likely is lack of coolant in the cooling system First thing to do is check the coolant reservoir. This will be a plastic tank with a cap that says something like "engine coolant" on it. Do not mistake the windshield washer fluid tank for the engine coolant tank. If this is empty buy some coolant and put it in to the full hot line and drive a little and check it again. Keep adding until the coolant in the reservoir stays up to the full hot line.

Let us know thru comments what you find when you look at the engine coolant tank.