Make sure you don't have any air leaks around the carb boots either-while running spray carb. cleaner on the boots (WD40 would actually be easier on the rubber) and if the engine responds at all you have a leak. The rad. cap could be suspect too, maybe the rubber gasket is worn out/cracked, it is an older bike. You could try a higher atmosphere cap (stock is 1.1 or 1.2 i think?) like what the KX500 runs, 1.9 atm. That would prevent losing fluid, but the bike would still be hot.
Make sure you're not jetted too lean (pilot circuit primarily-if you're on a lean main jet all the time you will most likely be moving fast, cooling the bike). Once you have all that stuff figured out, try running Engine Ice in the rads, it really does keep the bike cooler than regular anti-freeze. Technically, distilled water would keep the bike cooler- but it would also corrode the rads/motor up alot faster.
Good luck.