Hey Potomac,
what you are exolaining is exactly what is happening to my car too. The rpm surges while engine revving down at 3000 rpm even at a stop. I was thinking the idle control relay could be causing this because it is the only thing that can make the car rev up (my guess).
My car started doing this after I replaced the fuel pump relay. Did you change it too?
The OVP can also be a problem. Once I pulled it out when the car was idling and the car surged to 3000 rpm for 2 seconds and settled back to normal.

My car is 1989 230E. Please let me know if you discover something new about this.

__________________
1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
Sold:
1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
and few more american cars.