Before you purchase the new one, I suggest taking it all off and doing a really good cleaning.

This post: http://www.matthewsvolvosite.com/forums/viewtopic.php?f=12&t=29885&sid=473c1667b49c891f35f024a4e46e52f9

makes a lot of sense to me, which leads me to believe that the fault with the potentiometers (the physically contacting parts) is always to blame. To me it seems that when either of the two things fail inside (the other failure according to that guy is the servo that turns the throttle..) the ETM gets replaced. It makes sense, since replacing should totally solve the issue, though you might just be spending more than you need to. As I understand it, the physically contacting parts that 'fail' first are basically two potentiometers. Now the 850 has one in its TPS and usually those parts last 10-15+ years, which leads me to believe the redundancy volvo built in would not shorten the lifespan at all.

Anyway, if I were in your shoes I'd do a detailed cleaning and lubricate the shaft with some synthetic oil. The worst case you're out time and a teaspoon (or less) of oil, then you get the xemodex and you're set.