Just taking a guess here, but did the ABS light come on at the same time (your other post) as the idle prob?? I'd be taking a look a the OVP, check the 10amp fuse on top. Might need to replace the OVP also.
You should be able to find the OVP on the firewall 6-8 inches from the fuse box, Silver module with a red top and clear plastic flip top.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.