Samsung Galaxy note 3 (Dimming)

I just bought a 32GB Galaxy note 3 a couple days ago and played around with it some before running some benchmarks to see where it stacked up in the mobile market. Had no idea it was as powerful as it is.. But that aside I seem to have a strange issue, I have my screen set to Max brightness (NO Auto) all the time and I do not have the power saver option enabled. I downloaded an extremely demanding game called Sword Goddess which runs on the Unreal Engine 3 to really put it to the test, after about 5 minutes I will notice the screen start to dim. I thought it was a lighting feature in the game however after running Antutu during the CPU test it did the same thing. Is this normal for this kind of device, is it overheating possibly?

No its not. Running a screen on full brightness with auto turned off can and will lower the brightness. But I wouldn't expect it to be so early. Now considering you ran a very intense game that could of had a hand in it but shouldn't after just 5 mins. There are fail safes that will keep the screen from being harmed so it may be that.

My girlfriend and I both have the Galaxy S4 and she has hers on full bright auto turned off and mine is auto on and mine is noticeably brighter. Mine is about 1 month older as well.