Failure of the lift cylinders is not uncommon and the life of these is not particularly predictable. Replacements are not especially pricey, though I haven't looked into any for the 850 (but must soon). Since they are not visible, replacement is not as straightforward as on the 700 and 200 series. It is the sort of endeavor that a mildly adventurous amateur could undertake, but recruiting an assistant is probably a good idea.

There is, to the best of my knowledge, no spring contained inside. The action is produced as a result of contained gas under pressure against an oil-filled resevoir, just like a gas strut. With use the seal wears and the pressure escapes, crippling its ability to push, or in its extreme, even hold the gate open (binding really isn't the problem unless they've been bent or otherwise damaged). Cold weather does affect weak cylinders, a fact of physics.

Good luck.