Beetle turning over but just won't start

My wife has a 2001 1.6 VW Beetle for the last 2 years. Recently it has started to play up every 1 to 2 weeks. It wont start even though the engine turns over time and time again. If you leave if for about 2 hours it will start first time. I have taken it to 3 garage's but no one can seem to find any problems.

Any ideas what could be wrong?

Thanks.

I'd take it to a VW agent to see what codes have been logged or to a garage that can check for logged faults. This may cost but it will point you in the right direction. Apart from that I'd check the fuel pump relay and also check the drivers door latch which has within the latch a microswitch which tells the car if the door has been opened which then starts the fuel pump and primes it ready for starting. Maybe the microswitch is failing intermittently - check if the courtesy lights come on when opening the door.
When first opening the drivers door you should hear a hissing sound - this is the fuel pump priming the fuel system - no hiss and you have to crank till oil pressure is up and another circuit powers the fuel pump.