Well guys I'm glad to say my problem has been SOLVED! Cleaning and sealing the airbox has fixed my problem. I have put on 80km since doing the fix and the bike has the same amount of power as when I first put it back together.

I suspect the main culprit was that bolt hole in the center of the airfox section AFT of the filter. It looks like there's supposed to be a part bolted into that part of the cover, however I'v never had that part that it used to just be an open hole into the airbox section between the filter and carb.

So the carb really was getting dirty quickly after all. It also explained why I would have SO MUCH power immediatly after a carb clean, because my bike was getting much more air and running lean. In the first 10 km or so, I would be able to wheelie in first sitting normaly, without clutching or pulling the bars at all. I thought that was odd....haha. Then it would start to slow down, becoming sluggish and sounding whinney until it couldnt even hold 80kph after about 50km.
Now the power output seems to be more in line with what 15hp is supposed to give you and its staying constant. Quite nice actually...smooth and predictable.

Thanks everyone for your input and Ride On! The XT is a great little bike!

Glad you got things all sorted out. Enjoy the riding.

Good. A short piece of tubing, sealed on one end, is supposed to go on that nipple. It's there to act as as drain should you suck water into the airbox during a deeper than expected creek crossing, or oil should the bike spend much time running while laying on it's side.

^+1
I think of it as more of an inspection tube.

So the problem was NOT resolved. It just lasted longer than previous tries. I am not sure what to think anymore. I doubt it's an airbox problem anymore because after taking apart and putting back together many times over.....this thing is CLEAN.

With clean gas coming in from a new plastic gas tank and clean air coming through a very thoroughly cleaned airbox, I'm thinking it must be a carb problem of some sort. I had the fortune of finding a brand new complete carburetor assembly on ebay canada for 42$ and jumped on it. If it's a carb problem like I suspect, it will be gone.

If the problem persists now, I'm at a complete loss...

What kind of fuel line do you have? Gas with ethanol can break down different kinds of tubing which could be adding debris to the carb. Same with the petcock, it could be corroded inside and letting debris in. If you haven't already, install an in-line fuel filter and only use 100% gas.

I'll second Peejman's inline fuel filter recommendation. Once you have that new carburetor in place make sure the gas supply is filtered before it gets to it and you'll avoid further headaches. It's cheap insurance and easy to install..

Regarding inline fuel filters, avoid the cheap Chinese imports as the plastic will crack in the presence of heat and fuel. Alcohol in the fuel is always a good idea to avoid. Alcohol attracts water, is tough on rubber and oil.

I just got my new carburetor in from Amazon.ca. Delivery was nice and quick as promised. The carb has some very slight differences from the original but it was designed for the same engine and looks much much better than the old one so I'm sure it will work. I'll be posting the results when I get to chance to install it and give it a good try-out through a couple gas tanks.

Good deal Rocky. Looking forward to learning how the replacement carburetor works for you. Don't forget the inline filter while you're at it. It'll help you avoid future dirty carb headaches..