I haven't confirmed it yet (and you'll understand why after I explain it) but I was told that the Dell Client Manager agent that comes with the Dell Management Console was causing issues on some D620 units that had the Nvidia chipset. On those units the entire LCD stopped working and require Dell to replace the motherboard and LCD. While this series of laptop has been plagued by Nvidia related issues, I was assured that these units had been running fine for a few years already and the sheer quantity of them failing at the same time was unlikely. The current suspect is the SoftOSD component which gets installed as part of the Dell Client Manager Agent. I looked up this component and it appears to be directly related to video card/output manipulation which only adds to our suspicions.
Also, these units seemed to all have been docked for at least some of the time prior to their failure and caused odd resets on the external monitor.
While this hasn't been verified yet, I would recommend that you uninstall the SoftOSD component off any D620 with the Nvidia chipset until we know one way or another. I'm not exactly willing to offer up any of my D620's for testing sacrifice. :)