"sticking" 164 throttle

[Jeff Matson <jsm8@domain.elided>]

The throttle in my '91 164 L has recently been exhibiting
the problem of not returning to idle when the gas pedal is
released (stays up at 3000 rpm or wherever I've had it). 
I've checked the cable and it's fine (unhooked both ends and
it moves very freely, which it should as it's only 1 year
and 10,000 miles old, replaced last year when the original
did physically get stuck).  The throttle feels to move
smoothly, too.  When it sticks driving, a little tap on the
gas pedal gets the revs to drop back to idle.  Anybody seen
this before, or have hints on where to look for the actual
cause of the problem?  The symptoms seem mechanical to me,
but I can't find anything obviously mechanically wrong...

Jeff Matson
Harvard, MA
'91 164 L (175,000 mi)
'74 GTV 2000 (75,000 mi)
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