1998 Isuzu Trooper Hesitation

Car starts severe hesitation, spits, sputters after going @ 150-200 highway miles, almost like choking if it had a carburator. Read a little about on line and thought it may be the EGR. Cleaned the EGR and found that it was stuck but seemed to be working properly when I reinstalled but the problem remains. Car starts right back up after a brief stop but the hesitation begins again after only 70-100 miles. Never does this on around town driving, only when the engine has run for extended time. Has new spark plugs but did not replace the 6 ignition modules on top of the plugs. EGR or ignition? Could it be something else?