Most likely, your front (pre cat) O2 sensor is failing or has failed. Just went thru this exact thing with our 96 850. The sensor itself has a fault code but it doesn't usually set a code until it is almost dead. It's an easy DIY job to replace (see bay 13) On the sensor, (1) Use only Bosch sensors and double check that you have the correct one. Some vendors has a chart that lists the wrong sensor for some 850's. (2) Treat it like glass. It has a ceramic tip inside the "nose". Drop it and it's probably history. (3) don't get any of the anti seize paste from the threads on the sensor nose. Another possibility for your problem is a major vacuum leak. Bay 13 will list where to look,

Good luck

bl