It could be the fuel filter and if you don't know the history on it I would spend the 20 bucks for a new one and swap it out.
If you read the FAQs it mentions about how to test for a leaking intake manifold gasket :
http://isuzufaq.ibctech.ca/#faq5
Something that has been showing up even more lately on the new V6 engine, the intake gaskets are starting to leak and break. If the gasket is leaking real bad you can hear an air leak from the engine, almost like the airbox is not tight.
Using a can of carb cleaner you can check this very easy. Spray around the intake, most of the time on the drivers side, and you can find the leak.