Stuttering when coasting above 40mph

My '94 Voyager stutters when I am cruising at speeds above 40mph; but only after I have been driving for a couple of hours. It started doing this a year after I bought the van. Can someone advise me on what I need to have done to stop the stuttering?

I checked my trans fliud and found out that I had too much in. I remove about a quart and the shuddering only happens after I have been driving on the highway for more than 1-2 hours at a stretch. could this be something else?