Severe overexposure with DB600 on D90.

This is a recurring problem I've been having with the SB600 on my D90. Severe overexposure almost exclusively when taking pictures of people with the flash on the camera. The camera is usually in program mode with no flash or exposure compensation dialed in. When it happens and I notice it, I can't seem to fix it. Lately if I notice it, I've been taking the SB600 off and just using the pop up flash which works fine. Not the ideal solution though. At other times I can't seem reproduce it no matter what I seem to try and everything seems fine. It's getting old and I've lost a number of shots I would really have liked to have had.

Examples: