Heater issues 03 a4 1.8
 

A few months ago a coolant line burst on my A4 1.8. replaced it, the turbo coolant hose and thermostat. Also did a timing belt water pump change while I was at it. After all was said and done the heater wasn't working. So I flushed it out and it worked fine for about two weeks. I flushed it again and same thing. it blew real hot for a while and then it gradually, over the course of a couple weeks, began to cool off. To the point that now it only warms up a little while accelerating. At idle it's ice cold. Do I just keep flushing it or is there some coolant system mystery that I am unaware of? Thanks