'99 Civic engine cuts off

I own a '99 civic, front wheel drive automatic, currently 70000km, and I am getting a very strange problem, when I start up the car it works fine most of the times, and then sometimes it cuts off and I notice the fuel guage goes to empty (the gas tank is full) I will try to start it up but it will not start the fuel guage will remain on empty, sometimes if I leave it alone for an hour it will start and the fuel guage will go back to full. Then again sometimes it will not start for days. The fuel guage is the only thing that I notice on the instrument panel that is not working at the time this problem presents itself. I took it to my mechanic who works at Honda, and he says that everything is mechanically fine, so now I am evn more confused as to what to do.