On my 87 syncro gl I had this problem.  The 2000 idel seems to slow down once
the van is warmed up but besides that check out where the throttle runs into
the engine.  Where there is a spring that holds it back, check its stength,
mine was not strong at all and the throttle would not return all the way.  I
cleaned it all the gunk out of the throttle arean and then shortened the
spring up and all is basically good now.  I'd check this out and then move on
to perhaps your idel stabilizer...maybe that needs a good cleaning.  Hope this
helps.

Tim G
87 gl syncro(162k)