As Paul said mate, could be your ignition switch or reader coil playin up. I had a similar problem with my car, drove it, switched it off, then it wouldnt wanna re-start, say for instance I had to put fuel in!! Had to wait for 10-15 min, then it'd re-start. Problem was Immobilser on Clifford. Initially thought was Ign.Switch, Wasnt! Then reader coil-Wasnt! So then had the alarm checked.