Why does my horse quit on me?

I have a palomino QH who is 8 years old. When I got him he was sold as greenbroke, but I noticed he had forgotten how to trot and canter, the owner had never gone into a gallop with him. I re-taught him the trot and canter, and we gallop now. He doesn't buck when he runs anymore. One thing he will do is just randomly quit. We'll be walking and he'll just stop. I'll tap him with the crop and kiss to him. No response. I'll use leg and he'll just pin his ears. I'll start to do a circle and he just stretches his neck, on both sides. I know my horse really well. He's just being rebellious. My cousin tried using some spurs... He just kicked under his belly and refused. He's ridden as a Western trail horse and is such a sweet gelding. Any idea on why he stops? (His teeth and feet are fine.)