Did the original poster every solve his problem? I had a very similar issue, lights dimming, rpms dropping etc, but it wasn't a vacuum leak or WGA or EGR issue. I had just done a coolant flush and removed a bunch of wires and brackets around the coolant return. When I had replaced them I decided to toss that massive bronze bracket that is probably needed for the stock airbox or some other part I no longer have. I rerouted the harnesses and ground wires to what I thought were cleaner looking locations, however, I had too many in one spot with not enough contact to the engine. One of those was the fan motor ground, so any time the fan kicked on, lights dimmed and rpms dropped. The fan kicks on when you stop, so it seemed like an idle issue, took me a little while to figure out.
Rather than put all the metal brackets back in, I grabbed some 12 gauge speaker wire and routed it from where the battery connects to the chassis straight to where I was bolting the ground wires to the engine. Problem solved. Since the poster's problem is immediately after pushing in the clutch, it's probably not electrical, but poor grounding can cause all sorts of gremlins, so figured it was worth mentioning.