Hi Mike
I went out in June but took my own gear and stayed near Salo, quite a way down the west side, not the normal spot for windsurfing, so might not offer the best advice. I just got up at 5:30 each day to drive up to Limone or Tignale (or the road next to the lake near there) each morning to get the alledged guarenteed wind - it only blew 3-4 times in my 10 days there - had great fun when it did blow other than putting a hole in the nose within 5 minutes on the first day.
Europa holidays have a few places to stay and rent gear - they're just north of Malcesine.
http://www.europaholidays.com/en/index.htmlThere appeared to be quite a few places to chose from at Tarbole, but not sure on quality though. This seems to be the main watersports area too.
http://www.soulsurfers.info/garda/info.htmlthis page may help.
The windsurfing is generally at the north end of the lake in line with Malcesine and above and more so on the east side of the lake, with thermal winds blowing from the north in the morning and from the south in the afternoon. These did have a habit of dying very suddenly too, which i found with a 30 minute swim one afternoon with my 98l board. There wasn't even enough to float back, I'm sure a lack talent didn't help either.
Hope you have a good one - the Italians were pretty friendly even though I don't speak a word of their language.
Later
bob