It sounds almost like you have an air trapped in the system. When I worked as a radiator repair tech, we used to purge the system of air, by sticking a large funnel in the filler neck (it should be a snug fit into the filler neck, to allow coolant to rise into it). Then putting the heater on defrost, with the fan on high, warm up the vehicle until the thermostat opens. During the warm up, the funnel fills with coolant, when the thermostat opens it gets sucked down, add coolant to the funnel so it's level is higher than the filler cap, and rev the engine in short bursts and watch the air bubbles come up in the funnel. Once you figure your air is all out, bring up the rpm's steady until the coolant in the funnel is in the rad, remove funnel and cap the system, then let the rpm's drop back to idle. Done, works 99% of the time. Another thing we used to do to help get air out is drill a small hole in the "ring" area of the thermostat, if the thermostat didn't have one already. Could also be a new faulty thermostat or a faulty water pump, but the pump is new too so I doubt it's the problem..
