As I've been messing around with Ham Radio for some time now....If you think it's a thermal problem (when it gets hot) you can pinpoint items with circut board freeze spray or something like it. It will allow you to almost make parts "fail at will" if the parts problems are heat related. I've even made makeshift cold rooms and hit them with a tank of Co2 to prove out the problems. In a pinch a zip-loc with ice may get you by. OMI'm dealing with the same problem on my xt200...haven't been able to really ride it for years. It starts on the first kick, but after you let it warm up, It just cuts out and dies when you get into the upper revs. It won't restart until it cools and sits. I just put a brand new chinese carb on, but it does the exact same thing with the well cleaned TK on there as well. I think it's the CDI, so I'm trying that next, but I have the feeling it could be a bad pulsar coil, or even the main harness. I already swapped the ignition coil so I know it's not carb or coil. I'm thinking either CDI or the stator section at this point. Only dies when it fully heated up.