C220 CDI No start + cutting out

[wotsgoinon]

My C-class C220 CDI intermittently refuses to start from the key, all the lights are on but nothing when key is turned. After several attempts usually and pressing brake, removing key etc the car will suddenly start and run perfectly.
However recently I have experienced terrifying all systems shut down (twice on motorways) - engine cuts out, headlights, dashboard...everything goes down only to come back on after say 5-6 seconds...what's going on?
When the car is running all seems well, no hesitation, no black smoke, accelerates cleanly...I've got this sneaky feeling that the two symptoms above are connected.
Any suggestions gratefully received

 

[AMGeed]

I'd start by checking the engine earth and the battery connections.
Complete shut down when on the move is serious.

 

[dharam574]

Do you have the engine light on, best to get a diagnostic machine hooked up to check code, preferably Star machine.

Do you have the problem of starting from cold or warm starting?