Over heating
 

I have a 02 Audi A4 quattro 3.0 v6 with over heating issues. First let me start off by saying I noticed a small leak in my radiator so i used stop leak hoping that would work till I got paid. So far I've replaced the radiator, thermostat (and I tested the old one in boiling water to see if was functioning properly and it was) , water pump and timing belt expecting to see some build up of stop leak on the thermostat and water pump and to my surprise there was none. And checked the exit and return lines on the coolant reservoir tank to make sure they're not plugged and they're not and I also checked the heater core to see if it was plugged up but it's not either. But I did notice the coolant is not circulating like it should it barely has anything come back out the return line like it should when the car is running and up to temperature and like I said I checked the return line by blowing in the feed line and coolant would easily come out the return line when I did that. I'm frustrated and lost and going broke trying to figure this out! I need help! Any suggestions or ideas what else I could check or do!?