2000 XT600 Problems

[AMICI]

Hola-
First post here, and just want to let everyone know that if you are ever are in Central mexico and need some help or have questions, please contact me, I've been living here 4 years and riding 2.

Ok now to see if anyone can give me suggestions.

After putting 10000km on my 00 XT (20,000km total) without any problems, it suddenly started doing weird shit that no one can seem to figure out.
The bike runs like a champ for the first 30 minutes and then starts to lose power like it is not getting fuel. I give it throttle, but it sputters, doesnt respond and the rpms start dropping. As i give it clutch to down shift, the bike shuts off. It will start up again with the choke on, and ride nice for a couple minutes but then acts up again.
3 different mechanics have been through it, taking apart the carb, cleaning the tank, fuel filter, lines, jets, pitcock and everything else associated with fuel. I thought it was just mechanics doing a half ass job, but am beginning to think it is something serious. They all say it is fine, but the same stuff happens when the bike warms up. They have been through the electrical system and it is fine, and just keep scratching their heads and saying how weird it is. I have minimal mechanical ability, but am learning more watching these guys work. Any one have any ideas as to what could be happening?

I was hoping to take off through Oaxaca & Chiapas last Wed., and my buddies have already left me behind here in San Miguel Allende, Mexico.
Any help you could give would be awesome-
Thanks in advance-
michael