1997 Honda: runs rough on highway

My son has a 1997 Honda Accord EX and is traveling this weekend. At highway speed after engine is warm, the engine hesitates or runs roughly when he is trying to maintain speed at about 65-70 mph. Two mechanics, including a Honda dealer, could not find anything wrong and suggested he may have some bad gas, but he has gone 100 plus miles and added new gas. The car mileage is about 120k and is well maintained.

Has anyone experienced this or have any ideas about what might be causing this?

It is impossible to start guessing without knowing basic maintenance history. One of the first suspects is the fuel filter. Has this been replaced? Has anyone checked the fuel pressure?

But any other basic maintenance items can cause this kind of problem - spark plugs, wires, air filter. Coils ought to be checked too.

“Well-maintained” is frequently not really an accurate characterizaion when we delve into specific details. If the car has not had its spark plugs, plug wires, fuel filter, and air filter changed within the past 20k-30k miles, that would be a good place to start.

If bringing the car up to date with maintenance does not help, then I would look at things like a Mass Air Flow sensor (MAF) that needs to be cleaned or replaced. The fuel pump is another possible issue, especially if the fuel filter was not changed when it should have been.