Nikon speedlights keep dying

My wife has an SB 900, about six months old. It's spent half its life in the shop. About a month after purchase, it died during a shoot and had to be sent to the shop. It took Nikon over two months to fix it. And it's just died again. No idea why. Another Nikon flash we own, an SB 600, also died and had to be repaired; so far it's still working.

Anybody else seen this dismaying lack of reliability in Nikon speedlights? Are there any decent alternatives for use with a D300s and D90? Thanks.