Dell Power Supply Shuts Off

I have a newer Dell Latitude D800 laptop. When I plug in the A/C
power supply, the green light on the power supply turns on, but after
a few minutes, it turns off again. Shortly after this, it will beep
for about 30 seconds and then stop.
The only way to reset it is to unplug the power supply from the wall
outlet, and plug it back in again. This happens to the cord before it
is even plugged into the laptop. This is about the 4th or 5th power
cord and converter that I have
gotten from Dell, and it has done it on all of them. I've done a
search on Dell and Google newsgroups, but haven't found anything to
help. Any ideas?

Thanks very much!

Chris