'94 EGR problems

I have a '94 Grand AM GT 3.1L engine. when i drive it at highway speeds, or if i redline it, a service engine soon light shows up. I got the diagnostic testing done, and the mechanic said the EGR vavle was bad, but he also said that since the EGR is non-essential, it was pointless to replace it for the 300+ dollars it would cost. the engine light is really annoying, so one day i took the valve apart, and i found a gray sludge stuck in the ports to the egr, and in the hose leading to the intake manifold, but none in the hose leading from teh exhaust manifold. i cleaned it out, and while it seemed like it helped i still got the light every now and then, so i took it apart again, and the sludge had returned! where can this sludge be from? i dont want to buy another egr valve if the engine is just gonna clog it up agian. can anyone help me out here?
thanks!
Dom

Try cleaning out the throttle body really good, make sure all the carbon build up is out. Hope that helps

Well, sort of like Davy is stating, the throttle body clean is a worthy try. The EGR vents exhaust gas through the exhaust manifold back into the valve cover, where the gases find their way back into the air-fuel mix to get re-burned. That is a very simplistic view of how it gets done. One major reason for your sludge is A] too rich a mixture or B] the valve or the ported vacuum feed to it is getting plugged and allowing the sludge to build up.