First make sure the bulbs are seated properly in the sockets. If this doesn't fix the problem try switching bulbs with the known good fixture and see if it's the bulbs. If this doesn't help either return the light fixture, you probably have a bad ballast or loose connection inside.

I unplugged the light last night and tried plugging it in again this morning. It did stay on this time. But, when I went to check on it a couple hours later, it was off again. Does this mean it's overheating or something?

It still sounds like the bulbs are not seated correctly. When it is plugged up try wiggling the bulbs back and forth slightly to see if they come on. Could be a bad fixture, but sounds like bulbs not making good contact.

I told you how to check it and it sounds like you just completely ignored my advice.

If the bulbs are seated correctly and it doesn't work right, return it for exchange. Is it worth burning the house down? NO!

Thanks, everybody, for your advice. I had actually tried quite a bit of bulb wiggling before I asked for help. So I already knew that wasn't going to work. And the fact that the bulbs did come on and stay on in the morning, at least for a little while, implied to me that they weren't at fault.

The light repeated the same behavior this morning, coming on when I plugged it in, but only staying on for a short time before going out again. My dad thinks it's probably the ballast.

So I've replaced the light with the old one temporarily. Even that cost me a gashed finger when a sharp edge came in contact with my skin. Being a clumsy type, I have a hard time juggling these long lights, hooks, bulbs, and everything without breaking something or smashing the plants beneath them! Which is why I don't like to fiddle with them any more than I have to.