Laguna Engine cutting out

I have a Renault Laguna 1.9 Dci, the year is 2003. When i put in the keycard to start the engine sometimes i get complete electrical failure and then the keycard wont work, if i just keep persisting it will eventually work again and once the car starts the car runs perfectly, the engine never cuts out once the car is going it just wont start sometimes, there is no set pattern to it.
I took it to my local Renault dealer and the put it on their computer diagnostic machine and they say they cant find any fault ????
Any ideas .... Thanks in advance