Tonya-I had the same problem a few months ago.  Take a quick look on 
your alternator.  There should be a small double wire with a spade 
connector near the main alternator wire.  Mine was making a bad 
connection and this caused the exact symptoms you described.  I'm not 
saying that it couldn't be something else, but it's a quick, cheap, 
easy check.

Scott
'84 Westy