Sometimes it is nothing more than cleaning the battery cables and grounds. On my '95 850T I especially focus on the ground cable in front of the battery on the body. Clean it thoroughly. I use coke, a wire brush, then clean it off...maybe you would rather use something safer and less corrosive than coke...but it's worked for me. You can save yourself lots of money in trouble shooting by trying this simple task.

Good luck....dealers were stumped on this one...one wanted to replace my alternator....it is still fine several years later....

Clean that ground every 6 months or so...at least once a year.