400ex engine problems...please help

I will try to keep this as short as possible....the quad will fire right up and idle and purr like a kitten with no knocking, smoking, or ticking for about a minute or so, and you can even give it throttle and it will respond great, but after a minute or so it will begin to start cutting out and then die within a few seconds. Once this happens i cannot get it started. If I let the engine cool off and try again the next day it will do the same thing. I have adjusted the valves a couple weeks ago because of this same problem. After I adjusted them I went on a hard 3 hour ride at the track, then parked it for 2 weeks, now this. I have readjusted to valves, they were not out of spec by much, but this time, it didn't solve the problem. It's almost like it cranks and runs great, but when it gets hot, it will die. I haven't checked the spark plug, it is hard to get to, I will check it next. It really doesn't seem like it is timing or carb issue because it does crank right up and run great for a minute or so. Right before it dies it will smoke just a little, but other than that, nothing. Thanks so much for reading, please let me know if you have any suggestions, I'm thinking maybe my valve seals are leaking, but I'd love some other opinions.