Computer problem?

I bought this car last month (private sale) and drove it back to London from Norwich no problems. Drove it around London no problems and then put it in for a full service. After that I had to call the RAC out twice in one night when the car wouldn't start. First time the guy said he thought there was a problem with the computer getting confused about the engine temperature, the second time I was told the crankshaft sensor needed replacing. I duly replaced the sensor and after disconnecting and reconnecting the battery the car started. I booked it into a Kia garage for a diagnostic to double check everything was ok but 20 mins into the drive it just cut out and had to get towed the rest of the way. Now the car is at the garage who can't figure out what the problem is. Nothing is showing up on the diagnostic and the engine seems in good working order (aside from the small problem of not starting!)...any ideas anyone??