Speedlight flash either ignored or way too bright

Hi. For quite some time I've used a D90 with a Speedlight SB-600 but recently for no apparent reason things are going haywire.

With the camera set on Auto mode and the Speedlight flash ready, the D90 chooses to try to use the built-in flash instead of the Speedlight. That happens about half the time. The other half, the Speedlight is triggered, but the photo is very over-exposed.

It's possible I accidentally changed some setting either on the flash unit or on the D90, but I don't think so. At this point I don't know whether the problem lies with the flash or the camera.

Any advise or suggestions?

-Ken