I was having this problem on the drive side and took the bike to my LBS. They just replaced the broken spokes and trued the wheel. Two weeks later I broke another one. Took it to a different shop. The mechanic there was a roadie/racer. Told me that there was not enough tension on the spokes. This allowed too much flex, thus increasing metal fatigue. He tensioned the spokes properly. This was less than a week ago, so I don't have long term results yet - but I rode it 51 miles on Sunday. No problems. It also feels like I get a "just noticable difference" when I accelerate from a dead stop - perhaps because it doesn't take that split second longer to take up the slack in the spokes. Good luck.