dodge ram van 1988 b250 8 cyl 318 136000 miles

I recently purchased a 1988 dodge ram van. I am having a problem with the engine running improperly on an intermittent basis. The problem typically starts on the highway when i get the van up to about 60mph. It will suddenly start to lose power, as if the engine is not getting gas. It will then run smoothly again, but will go back and forth rough and smooth. I have about a 7 mile drive to work, mostly on the highway. The problem occurs about 50% of the time, when I drive to work. Other times it will run fine for several days, with no problems. Once the problem occurs, it will usually run rough for the rest of the trip, even after I get off the highway. It will often die on me, once I stop at a light or stop sign. But it will always start right back up. The problem does not seem to be weather related, as it will run roughly on dry days as well as rainy ones. I thought it might be a fuel pump or fuel filter problem, but in following the gas lines from the tank to the carb, I could not locate a filter or a fuel pump. Any suggestions would be very welcome.