This doesn't sound like typical Vanagon Syndrome but who knows. I used the
following temporary fix for Vanagon Syndrome but it only works with a stick
shift. When your Vanagon begins to lose power, simply engage the clutch and
rev the engine a couple times. This seems to clear something, maybe the
ECU, and if you're lucky will return you to a clear running engine,
although only temporarily. In my case, it returned my engine to good
running for anywhere from 15 minutes to 2 hours on interstate driving. Good
luck and welcome.

By the way, do a search on vanagon syndrome on the List so you know what
we're talking about.