Luke-warm start troubles

I have a 2000 Miata with 87000 miles. If it sits after running for less than 30 minutes, it starts up fine. If it sits for longer, but less than a few hours, it has trouble starting. The engine turns over fine, but will not fire. When it eventually does fire, it runs rough and smells of gas. It will run very rough for about 5 minutes and then instantly switch to running perfectly. The problem is intermittent. I've taken it to two mechanics, have had both oxygen sensors, the valve cover gasket, and all the belts replaced (with no effect).