Two additional pieces of info. When I am driving, if I accelerate
hard, I hear a slight leak-like sound for about 1 second then it goes
away.
It does not do this if I rev the engine in neutral, only when there is
a load on it.
Also, I read another post where someone mentioned that when driving, if
he stepped on the clutch the engine would stay idling at about
1500-2000 rpm as long as the car was moving. When he came to a stop,
the engine would drop back to a rough 800RPM.
I checked, and my explorer shares this trait as well. His problem was
with the Idle Air Bypass Valve. Does this sound like a good place to
start?

But why did this only start after replacing the manifold gasket?
-Dan