Excellent question! I wish I had an excellent answer for you. But......and I'm no expert by any means.....
This is what I have heard and understand about the EGR valve. The valve directs a small portion of exhaust gasses back into the inlet air charge and lowers the maximum temperature of the burning fuel. The valve then makes sure that the EGR system is off at idle, where it would lead to erratic and loping idle, as well as at full throttle, since adding exhaust gasses robs power from the engine. Therefore, maybe the valve is staying open??? Possibly the ECM is telling it to stay open? Have you checked your O2 sensors? Could there be a vacuum leak? Keep the EGR valve unplugged? I think I might be I'm shooting in the dark, but no one else has replied and didn't want you think we didn't care. If this doesn't help you, I hope you find the answer soon. These things can be a pain the butt. I wish you the best of luck!
If you figure it out...please let me know what the culprit was.