so, when i am driving my 99 s70 with 160k miles down the highway at 60 or so mph, and i let of the accelorator, the tach fluctuates between 1200 and 1800 rpm's, then finally settles to 1500. why is this? when i shift into neutral while examining this the rpm's immediatly drop to 900 which i belive is normal. it also idles fine at 900 when in park or sitting still in drive.

 

Sounds like throttle body/IAC valve problem, or a pretty bad primary vacuum leak. Try cleaning Throt. body, and IAC valve see if it helps at all.