Starting funny

[Bjeno]

First off, a bit of background, the car has the not so nice fuel smell when its running, mechanic has had a look, cant find anything a miss except the fuel tank may have a slight leak. But what i'm really interested to hear is your opion on this, the mechanic was really stumped. When you start the car more often than not fires up perfect but on occasion it wont start when you turn the key, but as soon as you let go, it fires. any ideas why? anyone else seen this occur?