HELP! 4.2L Surge @ 73 mph
 

I need Help determine the cause of a problem that recently started this weekend. I have a 1998 4.2L V6 reg cab with 33,000 miles. We had taken the truck 6 hours to Michigan. The day after we arrived on the second tank of gas my wife said the truck started surging for a few seconds. At that time she was driving about 50 mph and not accelerating. Then it didn't happen for another 360 miles and 3 days. On the way back from Michigan after driving about 5 hours straight with the cruise set at 73 mph it did it again it surged for a few seconds and then continued on. The second time it happend it was on a different tank of gas. I wasn't sure what was going on but the rpm's didn't drop very much and the cruise held constant. In the past I have had some slight spark knock however I usually change brands of gas and it goes away.Does anyone have any ideas? I would rather solve this then take it to the dealer since the warantee is up. Thanks in advance