Here is a great article on this subject
http://www.thirdgen.org/egrdiagnose
Now, more than likely it is a vacuum leak. In the article I linked, it talks about how at highway speeds, this is when this typically happens. If the vacuum line is broke or plugged, the ECM doesn't see an increase in temp when the solenoid opens (recirculating exhaust gases) and throws the code.
Happened to me yesturday. Pulled over, turned it off, started it, no light. Seeing how you've replaced the EGR valve 3 times already, I would be checking those vacuum lines. Some are brittle plastic and the soft rubber they go into get dry rotted. So dilectic grease massaged into them helps, but it's sort of a quick fix. Hope some of this helps you!
It would be so cool to have bought my '91 new, but I've got it now!
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