it's not my daily driver, I drive it once every 2 weeks so the battery die. But even after a 30 min drive it does not warm up and the idle bounces like crazy from 750rpm to 3500rpms. Do I need to check the coolant, replace the thermostat? If the car is not moving after 20 min it will get to the halfway point/operating temp, once it gets moving over 40mph the temp drops down.

 

First, double check to make sure the radiator is COMPLETELY full of antifreeze/water and that the overflow bottle is half full.

If the radiator is full, then the bi-metal thermostat is either stuck open or missing (or possibly upside down). Follow the top radiator hose back toward the engine. It is located under the first metal fitting you come to. Most of the time, it is just rusted or stuck open.

PS - the water flows from the engine to the radiator in this hose. The thermostat will have an arrow or something to show water flow or one side will say UP. If in doubt, replace it because they are cheap. ($12)