Does your 850 have a sunroof? If so, on a nice sunny day with nothing better to do you might want to pop out the mechanism covers (they slide toward back about a 1/2 inch off three little pins — my covers were jammed, had to drill a hole straight through the side, stick a needle nose plier in there and hit it lightly toward the back with a hammer). Then, take off all 3 screws on each side and pop out the glass. Check it for separation from the metal strip along the bottom edge (which would mean the seal is leaking along the side or back). Vac and blow out the slide tracks. I'm going to blow out my drain tubes from the bottom up through. Grit can get in the tracks and bind things up.

Clean up the mechanisms and re-lube cams, etc with a light lithium grease and some spray lube on all pivots. It goes back together pretty easily. Just note screw positions, but you can tinker with adjustments to get it reseated if the drive mechanisms are all working right.