2002 Chevrolet Express bucking or missing

This is a 1500 v8 vortec full size express van. Recently while taking longer highway trips the van will buck and jerk. It goes away for a couple of miles and then returns. On three recent 120mile trips the bucking started at almost the exact amount of miles or time. 40 miles or 40 minutes. I had the differential checked and fluid replaced and this had no effect. Was not sure if problem was in the front or in the back. On my last return trip I added some gas treatment. Bucking was not as frequent but still there. Have not run the remaining gas through yet. This van does not get driven regularly. Will sit in garage for one to two weeks at a time. Fuel pump replaced at around 90,000 miles. Another issue that I'm not sure is related is when we take our daughter to college we have to go up about a 4 mile mountain hill. 4 out of 10 times the van will studder and buck and CEL will come on and blink. It will stop blinking after we level off and the engine will be normal again. No codes have ever shown up. Hope you can give me an idea on what the problem is. Thanks