This type printer usually is good for a few years then repair, support exceeds replacement costs. I see it came out in 2004 so maybe it's time.
After several years of satisfactory operation, my HP 6840 has started shutting itself off every ten or 15 minutes.
I use it in Wi-Fi mode. I have two desktops (one running XP, the other VISTA 32) and two laptops (one running XP and one running VISTA 64) which print through this printer, using the Wi-Fi router and/or WAP node.
HP told me to deselect an option in the "properties" screen which provides an auto-off. Strangely, the option did not appear on most of my computers or was grayed out when it did. I selected "no" when possible. No help.
I tried correcting the situation by resetting the printer to the factory defaults and reinstalling the drivers. Still shuts off. I turned off all of the computers and Wi-Fi equipment, so that the printer was the only device on, and nothing was connected to it by wire or Wi-Fi. It still shut itself off after 15 minutes or so.