98 mx-3 runs but stalls going down the road
 

hello all I'm new to the mazda series and I just recently ran into a problem. A little while back my car dies and had to jump it. Thought it was fine, but now I gotta jump it every time but now it stalls when I'm going down the road when in neutral or when I'm just sitting still, checked the battery when running and its reading 15 plus volts kinda high I think. Disconnected the battery and it still runs...so it's not the alternator. Doesn't this seem along the lines of a voltage regulator and is it external or internal if that's the case?