1992 Cadillac STS Idling Problem?

Hello. I have a 1992 Cadillac STS with an irregular idle. It used to idle around 750 RPM which was nice. Now it varies between 500 RPM and as high as 1500 RPM (it's usually on the higher side). I did the self diagnostic on it and it comes up with code "E030" which the service manual says is... show more Hello. I have a 1992 Cadillac STS with an irregular idle. It used to idle around 750 RPM which was nice. Now it varies between 500 RPM and as high as 1500 RPM (it's usually on the higher side).

I did the self diagnostic on it and it comes up with code "E030" which the service manual says is "Idle Speed Control RPM Out Of Range". I know what this is, it's a little screw-like control that an arm sits against when you aren't pushing the gas pedal. The screw moves in and out but doesn't return to the same position everytime to give a constant and correct idle speed.

I've also heard that those codes are to be used as a guideline and are not necessarily what's wrong. Any ideas or suggestions as to what else could be wrong would be greatly appreciated. Thank-you in advance.
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