My son just moved up to a KX 80. He rode a 97 XR 80. It started throwing the chain on a regular basis. It drove me mad trying to figure out what was going on. Wheel, bearings, chain, sprokets, you name it. Till one day a friend and I were just looking around at it and he touched the lower motor mount bolt. It was loose. Actually it was busted and both ends were just stuck. As my son would ride, or jump the engine was shifting just enough to throw the chain off the front sproket. We replaced the bolt and it never happen again. And we all rode off into the sunset :)
check the actual engine mounts and alignment