Help motor goes off for 30 minutes and on again after

Hello,
i own a 2001 smart 4two cdi/diesel.

My car stoped after few minutes driving at a red sign and only started after 30 minutes. And i've been on that cicle for a month allready.
During that time the electrical part its working and when i turn the key everything's fine but the motor doesn't give any sign.

Now their saying that i have to change all the transmission. It's expensive and i have my doubts cause i dont have any problems in trasmission and it doesn't seem to be the problem.

Please help,
i'm desperate and the transmision money it's almost the comercial price of the car

Thanks
Paulo Inacio