eh2+b18c1=usdm sleeper

i know its still a lil early, but im trying to get my car ready for winter. im still trying to figure out a problem i had last winter, my heat is cool at idle and will only warm up if i stay on the accelerator. i tried replacing the thermostat and replaced and bleed my coolant last year.. it didnt help so im here looking for some answers.. ive tried searching and came up with "its your heater core" but i wanna make sure this is the culprit before i shell out a $140 one.. has anyone had these sympthoms what was it. TIA.