If the check engine light comes on each time you drive it at highway speeds, it's EGR related. Every time it comes on means a new event has occurred, not because a code was already stored. Usually you need to get up to road speed for it to light up. Clearing the code won't make it go away. There are still problems with the emissions system.
Test the EGR valve by disconnecting the vacuum line, then from underneath, press upward on the diaphragm. Plug the vacuum port with your finger and while still touching the diaphragm, release the pressure. If it moves downward, you have a hole in it. That indicates a leak in the system, just like a broken tube.
If that isn't the solution, then you have a problem with the EGR solenoid. I have several you can test fit to see if that is the solution.