My 96 Dodge Grand Caravan stalls after driving on the highway for a while. Once I've been driving at highway speed for at least an hour ormore, the van suddenly looses acceleration. I can make it go for a few minutes by playing with the accelerator, but then it "bucks" and looses all acceleration. It will sputter, engine backfire, and move sluggishly before stalling. It has a hard time starting after that, and doesn't run well. It will only start smoothly and run for a while after sitting for at least 1/2 hour, but usually for 1-2 hours. But, eventually, it will stall again after some driving. Any ideas?