What do you guys think is the cause of this?

I just bought a beautifull 88' 560SEL & I really couldn't be happier. The only problem that I've come across that concerns me is that the enigine revs way too high when it is in neutral, about 1750 rpm. I am confused because the car idles fine when in drive, but whenever you put it in park or neutral, shift to reverse, etc. the rpm's shoot up. So if I have it in drive & shift to reverse, the engine goes up to 1750rpm before it engages, making the car lurch violently. This can't be good for the trans., so I want to get rid of the problem asap. Any help would be much appreciated. Thanks in advance.

>>>okay, I did a search under flairing and it seems that this is a problem with the idle control valve. Now what? Is this a matter of replacement or adjustment? how much will this part cost, and can I do it myself? I am new to mercedes, but I am fairly good with cars.
--sorry for all the questions!