P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient Code

I have a 97 Venture with 160000 miles that has been generaly a good car. Other that the usual intake gasket problem (which I fixed myself) and a new transmission there have been no major problems that I could not figure out, untill now.

After a hard run on the interstate the car would make a strange vibrating noise upon accleration and the check engine light came on with the above code. I changed the EGR valve and the car ran fine--for a day and now both problems are back. I don't know if the noise and the code are symptons of the same problem, but they did seen to appear and disapear simultaneiously.

Any help any one could offer would be much appreciated.

Thanks