I know loads of people will be having this common problem at this time of year.
It usually starts with cold and damp weather.
Before you change your alternator, check this and save yourself some money!
Read through the 4 links.
viewtopic.php?f=54&t=39087
viewtopic.php?f=54&t=50183
viewtopic.php?f=54&t=37471
viewtopic.php?f=54&t=42211