I have this exact same issue, brand new printer( HP LJ Pro 400 M401dn ), out of the box, testing it before putting it beside its user. I thougt the error was due to the DHCP lease expiring when the printer was sleeping, but since you already tried a static ip I am at a loss.
I flashed it with the latest firmware, dated 20131214 before I noticed this problem.
When the problem occurs, which happend the first night:
Powerbutton flashing, Controlpanel unresponsive, will not answer to ping. Tried pushing the powerbutton once, Nothing.
Held down powerbutton, waited a bit, pushed powerbutton again, printer starts normally.
Please advise.