[merc81]

Took me a few minutes. . .I was looking for code 104, but as you said later it is code 401. Anyway, the pcm tests the egr system during a normal drive cycle. It does this by pulsing the egr valve open which should push up the MAP at the manifold. If the pressure isn't seen by the map sensor, it sets a counter. After about 12 failures, it lights the MIL light and sets the 401 code. Three things (outside of loose wires, bad connectors etc) can cause the problem: The MAP sensor is bad, which would also cause rough, seeking idle speeds, or the egr is bad, but you replaced yours (ouch, those are NOT cheap!), or there is some gook stuck down in the intake manifold from the exhaust gases.
I've seen the junk stuck down in the manifold. You might want to remove the upper intake manifold for a good clean out at the EGR valve location. This is much easier than it sounds, you just need to remove the stuff on top first. The gasket is reuseable. Or, you can get your head back in there and peek down the holes where the egr valve mounts to inspect for blockage. What you are looking for is a pebble size chunk of black carbon that blocks the flow of exhaust gas into the manifold.
John