Problem with Trustfire ST-50 going out on high

Help! I'm new to all of this and have a problem with my new Trust fire ST-50. (first half decent flashlight I've owned)

It has 5 settings, low, med, high, strobe, SOS.

When I put it in high sometimes after about 20 to 50 seconds the light goes off! This happens a lot!!

Sometimes after a while It just skips the high setting all together.



I'm wondering if it's the batteries or the flashlight?



Sometimes when it does stay on High for a few minutes. Say 4 or 6 minutes the high starts to flicker at different rates.



I'm using the Trustfire TR18650 2500mAh bats that came with it. Do I just need more batteries or do I have a defective light?



Last question if I do get more batteries what brand should I get. More trustfires? Or something else?



Allen