What would cause a surge at steady speeds in a Ford Explorer?

Both a friend and I, each driving 1995 Ford Explorers with the 4.0 L OHV engine, developed an off and on surging problem at speeds over 37 MPH. The problem was corrected by installing a new set of Ford Motorcraft platinum spark plugs. In my situation I only had 41,000 miles on the original plugs but 2 of them already had gaps wider than the specified range.