how much experimenting have you done? how long will the car run as long as you aren't going downhill? how steep a hill? Can you gas it if you're going down hill? Try going very slowly downhill without removing the clutch and see if it dies. I suspect that what's happening is that the transmission is actually forcing the engine over (kinda like how a push-start works), and when you remove the clutch, the tranny is no longer forcing the engine over and it therefore stops.