5 week old HPS is cycling. Help?

I purchased this light off ebay. It came with a 600w digital ballst and two HPS and MH bulbs. It's been running good for 5 weeks. I was just in my room watching my plant when all of a sudden the light turns off. It was close to lights out but not time yet. A few seconds after the lamp turns off it came back on. Its the 5th time it's done this. It'll stay on for a few minutes then shut off for about 5 seconds then come back on. Does this sound like cycling of the lamp or a ballast problem. I've only had this light for 5 weeks there is no reason it should be doing this.
Its an off-brand ballast, made in china.

Thanks in advance.

check to see if your bulb needs to be tightened. if it was the ballast, it prob. wouldn't come back on so quickly... use gloves on bulb, your hands can leave oils that can cause probs.

I just checked the bulb and it's screwed in nice and tight. I did something that might have resolved my problem tho. The light would stay on for about 5 minutes before it went out. The 5th time the light went out I went ahead and unplugged the ballast for about 30 seconds. Did a hot restrike and the bulb came back on. Now a longer period of time passed and the light didn't go out but then the timer reached the end of its 12 hour cycle and it shut off. Would a digital ballast ever mess up and need a hard reset to go back to its normal performance?

i think (think) they have to be off for a while after you turn them off and then back on. i can't remember.....

Today the light has been running good so far. I really wonder what caused it do cycle like that yesterday. Would a sudden drop in voltage throw off a digital ballast?

If a voltage drop go's down far enough, then yes it could turn it off competely. The Digital ballasts are more sensitive to voltage spikes verses the magnetic type.