84 528e idles erratically

Help, This is my first BMW it has over 300,000 miles on it but runs great... until last month. Now it is revs up and down between 1250 rpms and 1750 rmps when Idleing. I'm not sure what is causing this, I was thinking it might be Idle control valve but I wanted to see if there were any other sugestions before I tossed out $200 on one part.

Thanks for any help
Keith

You could try cleaning the IAC valve first. Carb or throttle body cleaner should do it. Work the valve manually a few times too. Blow out any excess.