Might want to check the cat to verify that it did not come apart internally.
I had this happen once and one of the chunks would block the exhaust causing a drop in performance. When it worked loose, things would be okay.

Todd
'88 Westy

 -------------- Original message ----------------------
From: Geza Polony <gezapolony@SBCGLOBAL.NET>
> This sounds more like an exhaust problem than anything to do with internal
> engine components. Check the exhaust flanges where they meet the cylinder
> heads. Is there a bolt or nut loose? Is there rust at that joint? Also--I
> had this happen--the O2 sensor can fall out of it's hole, making the same
> exhaust sound. This has nothing to do with ignition wires,  and oil
> additives won't help, either. My guess is that there's something loose in
> your exhaust system and when you stop the van  or mess with something else
> up top, it goes back together for a while. Hard to tell without seeing it,
> of course.