I live on a large hill and when driving down in the morning letting the engine brake I would go to accelerate and nothing would happen. If I let out the clutch it stalls immediately. After I get to the bottom of the hill I put the gas halfway to the floor and it just sputters barely enough to stay going and eventually it will stop sputtering and go back to normal like nothing happened . Drives fine up the hill and gas guage reads over half. Tends to happen more when it drops below half a tank but its never anywhere near empty. Any ideas on what could be causing this?