The 1st thing that comes to mind and the cheapest is there is air in the cooling lines. What I would do is put the front end of the car higher than the back (like on jack stands or hill). Let the car cool down for at least 2 hours. Take the radiator cap off and start the car. Let it run through 3-4 fan cycles and keep an eye on the coolant level. Might have to add some if bubbles start coming out of the system. Preferably the neck of the radiator fill should have coolant.

The other thing is the guy before you put a bad thermostat in. That is the next cheapest thing to try and easy to do.

The last thing that comes to mind is the fan relay or the fan is bad (this happened on my Civic once while in NYC in stop and go traffic). Since moving will force air through the radiator which in turn cools the fluid going into the engine.