[blazn240sx]

hi my friend just bought a car recently found out no thermostat ok so we added one right the car now overheats we try everything to fix it but no response ok what we change are, waterpump, radiator and cap, clutch fan, and upper and lower hoses the coolant 3 times bleed it about 5 times and now thinkin about changing the heater core because at some time when it overheat and we turn on the heater it doesnt work but when it at normal driving tempature it works so im stumped i dont know what else to change please give ideas and it only happens when we are driving it on the street through stop and go but on the freeway it fine