2005 Vauxhall Astra engine warning light comes on/car won't start at irregular intervals

We have a 2005 Vauxhall astra. The engine light has been coming on at irregular intervals for the last month. The car seems to drive ok for days and then the light will come back on. Sometimes the car won't start and the fault symbol will appear. This problem will also clear after a few minutes and the car will operate normally. The other day, the car stopped completely while out driving. Car would not start, however after a few minutes, the car started and has been running fine since. We took it to a mechanic, however we could not duplicate the problem. Any ideas what we should do next?