1990 80 quattro idle problem

[Jason]

I own a 1990 80 quattro that has developed an idle problem.  If the I push
in the clutch as I and coast as I'm slowing down for a stoplight or
something, the idle should drop to about 1100rpm and stay there.  However,
my idle drops to about 200rpm, catches itself, revs to 1500rpm, then drops
to 200rpm, starting the cycle all over again.  At stationary idle, the car
idles smoothly at about 8 or 900 rpm.  Could anyone tell me what's up?
Thanks,
Jason