It's a California van?  If the voltage is in spec when the engine is
completely warmed up and you're monitoring it (O2 sensor disconnected) then
it doesn't sound like a fuel issue and it doesn't sound like a vacuum leak.
Try driving it with O2 disconnected to see if it improves.  If it does, it
might be a short in the line from the O2 sensor to the ECU.  If not, then it
sounds like ignition issues, if you're sure the mechanical is OK.

From: "Greg Marshall" <Mr.Marshall@SYMPATICO.CA>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, April 18, 2007 3:01 AM
Subject: Re: unexplained power loss - help!

> The O2 sensor is brand new, and I just double checked it to make sure the
> voltage is in spec.
>
> Greg
> 82 Westy