APC 750 Smart UPS

APC 750 Smart UPS showing odd behavior. It&covering a single PC. With the batteries fully charged I pull the plug. It does the right thing and shuts the Win7 PC down after 2 minutes or so, still with a good charge in the batteries. I then restore the mains. The UPS is set to reboot immediately on mains being restored which it does, but then throws a hissy fit, flashing various lights after about another 2 minutes or so. The relevant bit is at 5 mins of video