Please replace that HPS bulb. A year is a long time for a HID bulb, and your yield will be low from loss of brightness from a lamp that old.

MOST IMPORTANTLY, there is a fire danger, a lamp on it's last legs will shut on and off like that, and in that stage there is an explosion risk. You could be seriously injured from molten glass, and also a fire could start.

Stop using the lamp and replace it as soon as you can.

Oh yeah, this is the wrong forum, next time try Basic Growing or something like that.