The intake manifold gasket leak is a very common problem. Isuzu put out a tech bulletin at some point changing the torque specs for the intake manifold bolts because apparently torquing them down to the original specs was causing many of them to leak.

If you want to test for the leaking gasket, it's very easy. Buy a 99 cent can of "starting fluid" from Wal-Mart or wherever. With the VX running, spray liberally around the gasket. If the RPM's go up while you are spraying in a certain area, then drop back down when you stop, there is a leak in that spot.