i have 2000 buick century custom, it started overheating when sitting at stop lights or going slow as soon as i moved it went back down, also there was no heat in the car, i loosed radiator cap

Unfortunately, it could be several things that can cause this (low coolant, clogged radiator, bad thermostat just to name a few). No heat can be an indication of not enough coolant. You didn't say if you have a 4 or 6 cylinder engine. The V6 on that car is known for blowing a lower intake manifold gasket which is a big job. You need to fill the cooling system and check for leaks.

no heat means no coolant if its too kow heater wont work. if its too low it will heat up fast especially when not moving when moving the air will blow on it and kinda cool it down so there is a leak that causes that some where make sure the cap is fitting and not letting air in or out then check hoses make sure no holes or leaks at connections points then make sure fan works turn zc on if it works it goes on. by the way never take the cap off when hot im guessing you learned that the hard way it dont blow up when you open if its out because theres nothing in there to steam. if the fan doesnt wirk then fix that. if it turns on turn ac off fill the coolant in the radiator and overflow when cold then turn on and watch if it gets hot and no fan on thdn its a thermostat issue if no fan its a fan issue if no visible leak ket it get hot then turn it off and check in a few minutes it could blow coolant out of the reservoir after you turn ut off that would cause it to have no coolant if the coolant isnt the same color as it was whdn you poured it in znd its briwn and thick thats a head gasket