2001 Opel Astra Car Switches off While Driving. Does not al

For quite a while now I have noticed the Speedometer needle just drop as if the car has been switched off while driving. This happens whenever I drive. On two most recent occassions the car has refused to start. I will turn the key and the enine will not even swing. After about ten or twenty minutes the car will start as normal.

-- Please advise