How do identify fault on Toyota Yaris?

The car starts fine. I place it into gear and it beeps and the gear indicator flashes. I place it back into neutral and the beeping stops. If I try and put it back into gear the same thing happens so I turn the engine off. The car then does not start at all. All lights come on but nothing happens.
Sometimes, when left long enough (usually after 45 – 60 mins) I try to start the car again and it is in full working order.
Happened 15/16 times in last 10 months, usually after a couple of short journeys straight after one another.
Garage unable to find fault

Michelle Vincent
January 2009