Optical tray won't stay open

Hello,

Dell 519 desktop, running Vista 64 bit. Having an issue with the optical tray. When commanded to open using either the button on the case or on-screen menu, the tray opens and immediately closes without stopping. This happens about 75% of the time, the rest of the time, it works normally. When it misbehaves, if I repeat the command, the cycle repeats as well, eventually after 4-5 times the tray will usually stop so I can insert or remove a disk. Once inserted, the disk plays with no problems. Any help greatly appreciated!