I had kinda the same problem. On my 750 sx, the engine had about a teaspoon of sand stuck in the water jackets causing cylinder one to overheat and seize. I would drive around and then it would just bog down and i couldn't go anywhere. once the cylinder cooled off enough, it started accelerating again. until finally the engine blew. I don't know if this is something to look for concidering you have a temp sensor and already cleaned the cooling lines?