Van drives great then stuck in low gear. Pull over start an drive fine why?

Is the speedometer working like it should??

I just went through the same thing with my 1998 Grand Voyager SE. It seemed like whenever I was in stop-and-go traffic, I would lose all gears but first. Sometimes it would happen while I was on the freeway. Pulling over for a while helped, but it seemed to take half the time after starting again for the same thing to happen. I learned that this is the van's way of telling you to get the check engine codes diagnosed. I had to get a transmission rebuild done - soft parts only. All my van's codes indicated transmission death. Final price, with a needed valve cover gasket included, was $2200. I have about 154,000 miles on it and the repair was worth it. New cars are expensive, and there is still a lot of life left in my old van. I hope you don't have a major issue, but sometimes repairs are cheaper than car payments.