1985 Volvo 240 STARTING

We have a station wagon and it runs really good since we had it 3 months ago then - last Thursday evening I get in to start it and it cranked but didn't fire; I tried quite a few times, looked under the hood and left it for a while - then I came back in an hour with no improvement. I needed the car for the weekend so got up very early the next day and had it towed to my mechanic - he called me a few hours later and told me it started first time.
Then yesterday (Thursday again) I came out of the movies at 9-30 and the same thing happened again. I left it for an hour and it still wouldn't start so I towed it to my mechanic and left it on his fourcourt - this morning he called to say it started first time - he had been looking at it all day but could find nothing wrong; what can it be?
I will say that this time I could smell gas when I was trying to get it started and on both occasions rain was in the air and I was in an indoor parking structure.
I'll add to this: each time it has happened the car had been inactive for about 2/3 hours; if it's inactive for an hour or so or so it starts or overnight; it's just that 2/3 slot that causes the problem.