Same thing happened to me last year. The mechanic identified the problem and had it fixed in a few minutes. Apparently it was one of the valves governing input of fuel to the engine ( sorry, I'm no mechanic, so not much more detail than that) which stuck intermittently due to accumulation of muck over time .Therefore, sometimes it would not start while other times, it revved up a storm.
The valve was replaced and no more trouble since. If your mechanic cannot idetify it, maybe it's time to check with someone more competent?
Good luck.
max.