yeah, whats the fan setup, and not running a stock overflow, or one in the stock location is totally stupid. (unless your car didnt come with one)

but if the fans are good, change thermostat, replace coolant with new, and distilled water.
bleed the air out, jack front end up high, turn heater on, use spill free funnell, or something to get the water levvel higher than the motor.
air bubbles like to hang out in the heater core.