I've been experiencing this stumbling with my 1990 4 liter for about a month. It only happens when I'm traveling on the freeway and hit a bump in the road. For the next few seconds, the engine would stumble, lose power. Then it would seem to run OK. Last week, it did it again, right after going over the same bump on the same freeway. Only that time, the engine continued to lose power, and I had to pull off the freeway. But once I got onto the surface streets, and driving more slowly, the engine ran fine. I got back onto the freeway, and was able to drive home with no remarkable incidents.

The last time I felt an engine behaving this way, its fuel filter was clogged. On this hunch, I replaced the fuel filter in the van. After driving through the same bump on the same freeway at the same speed, I did not notice the stumble as before. Of course, this is no proof that the problem was fixed. But the van is definitely running better now.

I actually replaced the original filter about a year ago, on the sense that at almost 150,000 miles, maybe it was due. So it was surprising to me that the filter needed to be changed so soon. Maybe I got a load of bad gas, or maybe I got a bad replacement filter. But I will keep an eye on how the engine runs for the next few months.