Change the alternator on my 93 850 this winter. I had similar symptoms, alot of draw would drop rpms. Bought a replacement Bosch from thepartsbin.com Changed out the alternator after removing most of the front side of the engine compartment. Prior to returning the core I took apart the brush assembly to see what was going on. The inner brush had cut a grove into the armerature about 1/8" deep. While the alternator had not failed, it was not going to be long before it did. Good investment.

Local autozones will also do electrical systems check usually for free. My local zone tested the system, and found the alternator only putting out minimal amps prior to replacement.