I had this problem and would have to recharge about every 6-9 months. I assumed it was the evaporator since the local gas station mechanic couldn't find any leaks in the engine compartment. Apparently the evaporator is located under the passenger dash and you'll need to look under the dash for the leak.

Anyways, I assumed it was the evaporator so I thought what the heck, I have nothing to lose if I try recharging it myself, I've watched the mechanic do it a few times so I know where the fitting is and read the previous posts here on this issue.

I purchased the Interdynamics A/C recharge kit with pressure gauge, R134a refridgerant and a sealant kit that had two cans, one for rubber (hose) leaks and another for metal leaks (evaporator). All for about $50.

Follow the directions carefully, I put in the metal and rubber sealants followed by 1 can of the R134a. The AC has been cool for about 2 years so far. BE VERY CAREFUL NOT TO OVERFILL.

Do a search on this site, there's tons of info about recharging (pros and cons). Unfortunately, this is a common and expensive problem for the 850. I figured I saved $1100 in the process.