I had a similar problem with my Vanagon,once,recently.Wrote about it here
on the list.Back in Sept,I think. Severly overheated muffler.Poor power.
Thought it was a plugged catalytic convertor.Wasnt.
Turns out that the next mornig ,when I started the Vanagon,all was fine.Up
to this point it sounds consistant with your problem.A temporary issue.A
Vanagon that had been sitting for awhile.

What I suspect was the  problem ,was that a mouse had moved into the
muffler and built a nest.The nest has now burnt down, and all is fine. But
up until the nest is burnt, you can get little power as back pressure
builds. The heat to the tin was sever. Didnt take long to expell the
organic material. Dont worry about the mouse.He is probably long gone and
up somebody elses tail pipe.Enjoy  your new Vanagon. Roger