ok for one, you cannot run without a thermostat, the coolant will never have time to properly cool and will always slowly heat up. Two you need to properly bleed your system, More than likely you have air bubbles, three, easiest step is to pull your spark plugs and check for any signs of rust type buildup or discoloring to them, if there is then its more than likely your headgasket causing the overheating which is a sign being it heats up faster when you get on it.
Here is what you can do with the engine running and warmed up feel in front of the fans on both sides of the radiator to see if hot air is being blown. if one side is hot and one side is cool and your hoses are hot then you definetly have an air bubble in the system. Rebleed the system till you get it right, if you dont it will overheat all day long on you because the coolant will either not cross the radiator properly or the thermostat will not open properly. Simple things you can try, its not rocket science there are only so many things it can be. Start with the simplest and cheapest and work your way up. If you have rust buildup on the plug or plugs then your headgasket is gone and that is causing the overheating. Im only running 1 1200cfm flexalite fan on my driver side of the radiator as a pusher and i live in the hottest part in the country, 116* days all the time sometimes over 120* and my turbo sentra has not overheated once. I use a 50/50 mixture coolant and water and that one fan on all the time. Thats it. And i guarantee my my Stainless steel tubular manifold lets off a lot more underhood heat than your setup does, although i dont know what your setup is. If its a basic bluebird/avanier/gtir then yes mine does.
Again start with the simplest and work your way up.