99 intermittent rough idling

I have a '99 Caravan SE Sport with the 3.8 liter V6 that is exhibiting intermittent rough idling. It started when I had about 50000 miles on the car. Up to that point it had been regularly maintained and ran perfectly. Then one day after a 100 mile trip, I noticed this rough idling when I returned. I took it to the dealer and they were unable to pinpoint a cause. They recommended I pay $700.00 for the recommended 60000 mile tuneup and that might fix the problem. I needed the car so I stalled on having the work done. About a week later, it started up and ran perfectly and there was no rough idle for months. Yesterday I returned from a long trip (450 miles in one day) and when I got into town I noticed the rough idle again at the first stop light. Now the van has 63000 miles on it. To me it behaves like something is getting into the fuel injectors periodically and fouling up their operation until it goes on through. Does this sound reasonable? If so, how can I test my theory? Any suggestions would be appreciated. Thanks.