Hi, I live in Denmark, so we should have about the same temperatures. Yours sound a bit warm, but nothing too serious. I've hit 120c on the interstate running only 100km/h in the summer. Nothing bad will happen. A fellow of mine once had a small-ish car with a oilthermometer (came like that from the factory). It "redlined" at 140c...

I know next to nothing about youre 5th gear, but it certantly sounds like you need to replace it. So on to the heat....

How long since you replaced motoroil, and which do you run? Have you checked that you have the right amount of oil? Running low should make the temperatures climb. I use 10w50 fullsynthetic from www.louis.de i use theire housebrand "Procycle" and change it at 3500-4000km, but looking at the oil I would guess I could go way beyond that, but oil is cheap, a motor is not. Better safe than sorry as they say.

How about valves, how long since last checked? I would check them just to rule them out.

Do you have stock gearing? Smaller front sprocket or bigger rear will make the bike rev more, creating more heat. Even a big rear tire can alter the gearing some. My big ass 17" knobby is actually as big as a 18" stret-tire.

Lastly, how about jetting, is the bike stock and/or have someone fiddled with the carb? Th xt's tend to run rich to cool the bike, but they're not that rich in the idle curcuit, so perhaps an adjustement of youre mixturescrew could help a tad.

You could also have an airleak around the carbs, leaning out the mixture a lot, creating more combustionheat.

It may sound like much, but most of it is quick to test and/or fix.

Good luck.