UltraFire C8 problem

Hello! So, about 2 days ago I figured I would get one of these cheap chinese lights. I got the ultrafire c8. I got it, charged the batterys, and it was great! I was really amazed! I put it up against my jetbeam pc20 and it was almost brighter than that! The second night I went out to try it and I turned it on high and it shut off within 5 seconds ... All the other modes work fine. It's just that high shuts off by itself after 5-10 seconds. I don't know what's causing this. I am using the stock batterys that came with it. I haven't tested other batterys yet. I'm just wondering if it is the light or the batterys or what's wrong with it. If you can help me that would be great! :help:

I suspect the 18650. If it's protected, it's shutting down. If it's not a good battery, the light will shut down. Try another battery. I like AW batteries.