Christopher, I think that I have found the cause (and solution) of one kind
of surging problem.

First, a bit of history:
My idle was starting to show erratic behaviour. When the engine warmed up
the idle would surge.  After a hot long drive, on occasion, we would  lose
the idle completely.

I followed Joel Walker's suggestion:
I cleaned the idle stabilizer with throttle body cleaner. I took the unit
off and sprayed the inlet and outlet until clean then used a small screw
driver to hold the valve open while spraying cleaner through the inlet and
out the outlet. After putting all back together, I took it for a test drive
and all ran perfectly ... well almost perfectly ... it still had a pesky
surging of about 25 rpm above and below 950 rpm about ever second or two.
:-)

The next day, I had NO IDLE at all,  NONE, KEIN. :-(

On examination, the idle stabilizer was humming, as it should, but
apparently the valve was not opening.  The only thing that I could think of
was that the throttle body cleaner had stripped the idle stabilizer of all
of its lubricants and the valve was sticking. With the idle stabilizer
humming (engine not running), I pulled off the inlet hose and sprayed a
light oil (Slick 50) into the stabilizer inlet, put the hose back on and
started the engine. It IDLED PERFECTLY with no surging :-)

The next day, it IDLED PERFECTLY with no surging. :-)

Today, we're going for a nice long drive and I'll report back. Larry and
Fritz ('86 Syncro Westfalia)

Q: why does the idle stabilizer hum? A: because it doesn't know the words.
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christopher irvin wrote:

> My  �84 has an inconsistent idle speed... it revs (or surges) every 3 or
> 4 seconds.  <snip> Does any one have any thoughts? <snip>