Current loss in Renault Scenic 1.6 gasoline.

Hello. The problem I'm having with my car is that it keeps losing power, but the strange thing is that it only happens occasionally. Even when I'm driving, if I stop at a yield or stop sign, the car stalls and I have to keep restarting it. The worst part is when I leave it in the garage the next day and the battery is dead. I took it to the mechanic and he jump-started it, checked the battery and alternator and they're fine, and the diagnostic machine didn't show any fault codes. Could someone help me? Thanks.