My 4.2 idles extremely rough after getting off the interstate when i travel a significant distance. Its done it for a while but i decided to fix the problem and went to ask my mechanic. Since i wasnt able to duplicate the problem i described it and he said to replace the EGR. He said hes seen this problem many of times on the v6 ford truck. Also, i would say that 99.999% of the time this guy is right on the money, so ill give it a try and see what happens and report back.