Most likely a head gasket leaking. Water pump, heater core, radiator, freeze plug, hose, fan inop causing boil over, etc..........
........from........????????
This is one of those yes and no questions. Yes: The sensor could be sending bad info the ECU telling it its cold or hot and depending on that answer could lead to huge pressure problem resulting in a break in a line No: You may need to replace hoses/coolant tank/sensor
It could be symptomatic of a larger problem. The coolant may not be circulating properly, or the indicator panel or car's computer could be developing trouble.
A cracked head could be the cause of the problem. This, along with a cracked head gasket would allow engine coolant to enter the oil supply. Coolant and oil do not mix well and over time, the oil looses its capability to lubricate. Check your oil for white spots and check your coolant reservoir for oil residue.
Anything could happen when a rabbit disappears.
If the problem disappears when the shifter in placed in neutral it could be a transmission or engine mounts
The coolant leak is pretty cut and dry - you have a leak somewhere. The lukewarm heat (assuming you're referring to the car's heater) could be for a number of reasons. Insufficient coolant could be one of those. Your coolant could be burnt out. It could be a problem with one of your heater hoses. You could have a worn out heater core.
try unpluging the sensor in the overflo bottle. it could be dirty, or no good
You could possibly have a circulation problem, or a over heating problem. Need more info.
Intake manifold gasket
It may be out of the coolant or a bad wire. The fuse could be the issue as well.
It could be but most likely the engine is overheating. Could also be that the cap is not holding pressure and needs to be replaced.