The microsoft 3000 wireless is actually pretty nice. It's a no frills wireless optical mouse. Gets the job done. Pretty good response time too.
http://www.microsoft.com/hardware/mouseandkeyboard/productdetails.aspx?pid=070
Most likely the capacitive reactence flux muster. They seem to have alot of problems with those going bad. Also check the sperving bearings. Seen those act up a time or two. I'd go ahead and replace them both.