What size hoses are connected to your motor from your pump. You may not have enough valume of water to your motor due to small water lines, like -6, -8, at low rpm. also check your thermestat. sp, if you have one. Is there a bypass relief valve in your cooling system. jet pump at high rpms put out tons of water pressure. If you using old style hoses like heater hose, there not rated for high pressure and or have a cracked hose on that only leaks when the pressure builds up.