Greetings. Trying to diagnose an issue that is this: when I'm on the highway my Previa will make a clunking noise and stall out for a brief moment. Like a loud, hard shift you might hear with an automatic transmission. This happens about every 20 minutes or so. It does not stall the van, it just happens and the van keeps going at the same momentum. A friend suggested pouring a cup of rubbing alcohol into the fuel tank when I fill up to see if it dries out any water in the bottom of the fuel tank. I'm worried its something more major. Van is a 91 manual alltrac with 212k miles. Any help appreciated.