2002 Pontiac Montana Code PO301

When I start my van in the morning it runs fine but after it gets up to operatin temperature and occasionally when it is damp out I get a missfire on cylinder #1.. I have had new platinum plugs, ignition wires, air filter, fuel filter and PCV valve installed on the vehicle. I have also gone to and automotive shop to see if they caould find and diagnose the problem.. They have determined that there is no problem with the wireing or plugs, there is an excellent spark going to the plug and feel that it is not an ignition problem . they also said that the #1 cylinder also has good compression. Has anyone had this problem before, if so what was the problem or does anyone know what could be causing this problem.

I only experience this problem when the engine is at normal operating temperature and somtimes it disapears after a few hours.

Thanks