The symptoms you are describing sound like your head gasket is letting combustion pressure leak past it and pressurizing the cooling system, but not letting it bleed back when the engine cools. Try popping the rad cap off, start the bike and look for bubbles in the coolant. If there are bubbles, order a head gasket, pull the head and check it for trueness. If the head needs truing up, it can normally be cleaned up with sandpaper mounted on a hard, flat surface that is perfectly straight, like a thick steel plate, or a thick piece of glass. The top gasket surface of the cylinder could need truing too.