I had that problem on my 94 Lightning. The check engine light would come on every so often, I would have to un-stick the plunger in the EGR valve. I eliminated the smog pump and associated equipment, and when I removed the intake to get to the plugs in the head, I noticed that the intake runners were very dirty, and I think that the gasket on the coolant side of the EGR spacer was leaking, causing coolant to go into the EGR valve. I had another problem of coolant going somewhere, and couldnt find out where.
So far, I have not used any more antifreeze. Maybe that was it, but I cant be sure cause with the smog equipment gone, the light come on all the time......
Which is my problem, how do I get the EGR out of the check engine light system....
BTW, I will post a step by step list of what I did to get rid of the smog pump when I am sure everything is ok..... after 800 miles, so far, so good....