2002 3.0 liter 6 speed awd a4 Quattro

Let me start by saying, previous owner ran 'the green coolant' for a LONG time. Since before I bought the car, it has "boiled" coolant, or more specifically it overflows very slowly from reservoir cap. The reservoir and cap were replaced a few weeks ago by p/o in an attempt to fix, but still does it.
When I got it, the first thing I did was drain coolant, and use banks cooling system flush treatment as directed. After the flush, I topped it off with the correct "orange coolant" and 'burped the air from the cooling system with the engine idling with reservoir cap off, and the problem hasn't gotten any better. I've did a bit of research, and discovered many people had same issue, and the problem was along the lines of; bad thermostat, broken waterpump impellers, and bad headgaskets.. tomorrow I'll be doing t-stat and while its out I'm going to attempt to see if the w.pump spins freely to see if that's the cause.. any other suggestions or helpful tips? Still new to the Audi scene and not very experienced so any input helps! Btw; car does NOT currently overheat..