the thermostat is probably getting stuck closed. i would start by putting a new thermostat in since its a cheap 5 minute job. if that doesnt fix it, then it might be time for a new pump, if it still doesnt fix it, something may be clogged up in your cooling system. im pretty sure you have to take the timing belt off to change the waterpump (ive always done both at the same time, so ive never tried to take a water pump off without the timing belt, and i think a timing belt tensioner pulley gets in the way) so if you arent comfortable with doing a timing belt, then basically dont touch it. i would start with the thermostat....thats honestly my best guess. it sounds like you are getting some flow, but it may be limited, because if you were getting no flow, turning the heater on wouldnt do anything.