Pushing a XP-E2 a little harder than usual and I noticed it starts dimming after a few seconds on high. Does this mean its overloading the emitter?
Likely overheating due to poor heatsinking. Depending on how hard you are driving your emitter, the thermal path may be insufficient, and the heat is building up, which would cause output to decline. Make sure your thermal path is good, and if your emitter is significantly overdriven, try using a DTP copper PCB. It could be a poor pill/PCB junction caused by a hollow pill, uneven surfaces or missing/inadequate/excessive thermal compound.
Other possibility; if your cell is of particularly poor quality, could be some voltage sag.
Use article silver thermal paste. Pill is aluminum, not hollow, on a notigon mcpcb. Pushing it at about 2A, which I didn’t think was to crazy.Usually I run them at 1.4- 1.5A’s no problem. I didn’t screw the mcpcb down because I was only testing it.
<Shrug>
Sounds like a pretty solid setup to me! Are you using a good cell to drive it? I had a poor quality UltraFire cell once (Once!) that couldn't hold up under heavier loads...
I get that problem sometimes. 
If the dimming is slight, I’d look to heat sinking first. The torch body should get warm rather quickly. If it doesn’t, you’re not getting heat away from the LED fast enough.
OTOH, the problem I keep having is, sometimes the Driver comes loose, sometimes the Pill unscrews itself; either of which makes for a host of problems, the most-noticeable of which is flipping itself into another Mode without my touching the switch. If your mode order is L-M-H, you’d notice the change. Question: when it gets dimmer, can you whack the torch on the palm of your hand & get it to flicker? If so, make sure the Driver is secure in the Pill and the Pill is tightly screwed in.
I’m assuming you’ve done the “usual” troubleshooting…
In any case, this is the best place on Earth to ask such questions, so let us know what you find & most of us will be happy to help!
Its not changing modes on me, that I know. The color seems changes slightly I notice. I am using a Panasonic 3100 mah. I can put the same battery into a headlamp that im running over 4A no problem. I agree it may be a heatsinking issue. I guess ill have to take a look at it again.
...or make sure the emitter is properly seated on your Noctigon? I've not experienced this, but I've read reports from others here where an emitter isn't seated properly. That may affect heat transfer...
<edit>Seated=flowed; emitter may need to be reflowed to ensure proper contact.
I will look into that.
XP-E2 voltage voltage is high at 2A, which is with 100% more than the specified 1A max. You know your setup (cells also), so I am saying that just to be take into account.
I had always heard running them over 1.5A’s was about max and anything over that makes no difference. I recently presented a graph showing they peak at 3A’s? Idk. Thought I would push one a little harder as an experiment.