Steve,
I have the EXACT SAME PROBLEM.  And i'll bet you something, do a simple oil
change and the ran will not have power issues for another 300 miles.  I'm
in the middle of troubleshooting my van (86 wbx), but have not found the
problem yet.  I thought i'de chat with you about it anyway though.  My van
does the exact thing with the oil pressure warning light/buzzer.  I
installed an oil gauge, and noticed something.  Ya know how there's an LED
in the center of the temp gauge?  When the needle is to the left edge of
the light, the pressure is fine (30 psi).  at the right edge, the pressure
is 20 psi (and ultimately goes way lower on deceleration, enough to make
the buzzer go off).  The happened so much i could guess the oil pressure
based off the temp gauge.  Also, after 300 miles after an oil change, the
oil pressure goes low more often.  after about 400 miles, i start losing
power.  I recently (100 miles ago) put in 20/50 oil, so do not yet know how
that effects it.  The ONLY theory I have is that my oil cooler is not
working.  Maybe the oil is getting hotter then it should (i do not have a
oil temp gauge yet).  My question for you is this:  Do you know the
condition of your oil cooler?  As well, what's the air temperature where
you drive?  It hovers around 100 where I live.   My last question is, what
does your oil look like after a change?  After 300 miles, my oil looks
dirty in the shade, and has a metalic sheen in the sun.  No large metal
chunks though, i tried filtering the oil using papers but nothing filtered
out.  Do realize this is a freshly rebuilt engine (<2000 miles)  Does your
oil get dirty fast, like mine? or was it still decently clear upon your
change?  Anyway, i cc'd this to you, so tell me some more.  Thanks.
Marshall

At 06:56 PM 7/15/2004 -0400, you wrote:
>Several years ago I had the pleasure of rebuilding my wife's 86 with
>Robert Lilley, who replied to a query I had on swapping engines.   My
>wife's van has run so well I have been off the list and out of the VW
>repair business for quite a while.  I had the great idea to buy another
>vanagon 89, and yes I am back in the repair biz.  Heres the problem
>...  The first 300 miles it ran great, very cool, well below the half way
>mark on the gauge.   Then it started getting hot, then the oil buzzer came
>on.   I have traced the oil buzzer down to  the high setting.  Coming off
>the highway when the RPM's drop below 2500 it goes off but will stop if
>you tap the accelerator above the 2500 mark, like it gets stuck
>momentarily .  Changed the oil to 20/50 put on a mann filter, still runs
>hot.  Changed the thermostat, both fuse relays and 70amp lead
>strip.  Droped the radiator and had it flush tested???  Not sure the
>competency level was there, but the tester said it seemed restricted, no
>exact science.  Changed the fuel filter, awaiting a new fuel pump
>arrival.  Any suggestions on what to check first on the heating up
>problem, and also the oil buzzer.
>With 4 kids 17 on down my wife will not consider driving anything else,
>therefore I am the VANFANMAN by default, when mommas happy ...
>THANKS