1999 Honda Accord Engine intermittently hesitates

About 4-5 months ago I was driving on the highway when my engine suddenly acted like it lost power for a few seconds, the jumped back up again, repeating this sporadically the rest of the trip. It didn't repeat this for some time. Over the months it has gotten worst until now it happens every trip. It deems to be OK at idle and just starting out, but primarily acts up when cruising at 70-80 mph. If I hold the accelerator steady, the engine speed and ground speed just drop off as if either the fuel was cut off or the ignition was cut off. Any movement of the accelerator will generally cause it to pick up again. If I kick it into passing gear when it is hesitating, it generally stops the hesitating, but I obviously can't do that very long. I drive 34 miles each way to work and most of the trip now I am pumping the accelerator to keep the car going. Occasionally it will suddenly line-out and run just fine - for a while. My mechanic says it is probably the transmission (which I replaced at 160K miles) or the distributor. He's leaning toward the distributor. Any ideas? I have put several cans of SeaFoam gas treatment through the tank recently and haven't noticed any difference.