Engine cutting out at speed

6 months ago my car had some major works, upper/lower dephaser pulleys, cambelt, sensors, battery and coil packs and a reflash of the computer amongst other things etc to the tune of £1200. Within a few weeks the engine cut out whilst driving and wouldn't start - was told I needed a new crank shaft sensor ... I had that replaced and the car was fine for a few months then it cut out again and wouldn't start for about an hour and then fired up and drove perfectly until it cut out again.

This time was told the connectors joining the crankshaft were loose and so these were tightened and the car has been fine for last couple of months. The other day, I was driving on the motorway for about 20 mins at 70mph and the engine cut out. I pulled over - it started fine and drove the rest of the journey home. I left the car for about an hour-!; when I went back it wouldn't start at all although it tries to turn over.
The garage came out about 6 hours later and the car started perfectly fine, the diagnostics read no faults at all so there was little he could do.
I drove last night again for about 20 mins on motorway and it cut out again .... Pressed start and it was fine until another 10 mins and it cut out again but when I pulled over it started fine and I managed to get home
What could be causing this? The garage said yesterday maybe the crankshaft sensor is the problem as that often fails when the engine is warm.... ? Surely this is still under warranty ... Could it be loose connectors again... I've asked the garage to take another look today, but if it starts and there's no fault to read - what can I do?

Any ideas what's wrong?

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Engine cutting out at speed