You need some alignment work or suspension problems - I'd take it to a place that specialized in alignments and tires like American Tire Company and have them check alignment and balance the tires. If you need to buy new tires you can get them at www.tirerack.com and have them installed at ATC. I recommend the Kumho Ecsta tires for the 850, they are far cheaper than the Michelins and at least as good if not better. I had them on my 850 for 2 years and they still have lots of treadwear left. American Tire Company will also offer you lifetime rotation and balancing which you should do every 6000 miles (not just rotate).

Have the sway bars, ball joints and struts checked at a reliable Volvo mechanicc as well.