Fiat Punto engine stops suddenly whilst driving resulting in a flat battery.

I am having great difficulty diagnosing what exactly is wrong with my car. It has been in the garage for over a week now and they are as baffled as me. My car drives fine and then all of a sudden looses power and cuts out completely flattening the battery. My car will not start again and have had to wait to be recovered and jump started to get it working again. The car will work first time after being jump started but then cuts out and i am once again left with a flat battery. Since my car has been in the garage they have done several tests which have come back to show the altinator and battery are both fine. However, they have now informed me that my heating has stop working. prior to my break downs i noticed that the radiator is loosing water with every journey and has to be filled up regularly. althogh there is no leak under the car and no signs of immulsification in the oil. Any ideas what might be wrong with my car as i miss it greatly and i am not fond off long bus journeys with my toddler. Please help !!!