Fiat Bravo 1.4 starting problem

Hi there. I have problem with my Fiat Bravo 1.4 ?98. The problem is that the car decide on it?s own when to start. Trying hard to start the engine but it doesn?t respond, there is something that stop the fuel and electricity of the car. Leaving it and trying again after half an hour or so it and everything is ok...weird thing. Where do you think is the problem, people? I took it to complete test at Fiat service-station but they say everything is OK. Do you think it can be something wrong with the injection system or maybe low-quality fuel? I?ll be delighted by any ideas. Thanks :)