Vauxhall Astra car won't start

I bought my car about 3 weeks ago and straight away drove a 3.5 hour journey and back again with no problems. A few days later the car wouldn't start. It is trying and cranks ur no luck. It now decides when it wants to start and when it doesn't. It's almost as of it starts when the engine is cool but then stalls and won't start as soon as the engine warms up. I have had it to the garage 3 times. Each time they say it is something else and I get it fixed just for it to happen again the next day. So far I have had 2 relays fixed, the fuel pump and the crank shaft sensor. Does anyone have any idea what this could be? Thanks in advance.