my car overheats mainly going up big hills. i keep topping up the coolant and it keeps draining, but i cant see any leaks ?where is the coolant going?

Looks like MAYBE head gasket trouble. 

The coolant might be getting into the combustion chamber. However, you might see white smoke from the exhaust tailpipe if it was bad enough.

  It could be an intermittent problem. Also, a bad head gasket can also push high pressure combustion gasses into the coolant, and force coolant

 out of the overflow tank and ultimately onto the ground, and by the time you check it, it is cooled enough, that it is no longer bubbling over into the overflow tank.

  and any evidence of overflow is evaporated. Hence, low coolant, with no evident or obvious leaks.