Preacherman wrote;
To investigate this "classic" problem, I invite Preacherman (owner) to be a little more specific in some areas. Although you are doing pretty good for an owner, terms like "My mechanic replaced the timing stuff" don't really tell us much.
You got a lot of good advice from the long time posters on this site. Be sure to check what they have said.
My two cents worth; At 180K miles, I'm going to dismiss the often very important statement that you smell coolant inside the car (or at least that is what I think you were saying/meaning) as not being a symptom of the actual problem, but the result of overheating and old plumbing inside the passenger compartment heater equipment (slight leak in heater core, loose heater control coolant valve, leaky hose when pressure or heat goes up, etc.)
I hate to see parts replaced as a diagnostic procedure unless there are no other tests available. Whether the radiator needs to be replaced is a pretty easy decision made by a radiator shop. It either is clogged, or it is not, or maybe aged and not up to par with a new radiator, but did it really need replacing? I am getting at the $1200 being spent.
I think you implied that the timing belts were already replaced with this current problem and the problem did not go away. So I doubt that you had skipped a tooth on one of the old timing belts (there are two, that's why your statement is so difficult) and then your mechanic replaced the two belts and got one of them exactly off by the same one tooth retarding timing and wound up with the exact same problem, but I've seen stranger things. So this kind of advice from the contributor above is good advice, but I would not bet on it if the mech already replaced the timing belts (not sure from your statement) (but still keep it in mind).
Was the lower radiator hose replaced? If so, was it replaced with a "non-collapsable" type hose? An old problem with lower radiator hoses is that the water pump causes a suction from the lower hose. If the hose becomes soft or is not a non collapsable type, then when the pump spins faster, the suction becomes greater until the hose collapses and cuts off water flow. But at idle, they almost never collapse.
Also you said that "but I have noticed antifreeze inside the engine compartment and the overflow is usually full with the cap popped off." When a large hose like a radiator hose collapses, the cooling system can not hold as much coolant and possibly this is why coolant is being pushed out the overflow. But it also makes me wonder about the head gaskets. If your Mech knows about the CO2 test, then he probably knows how to double check the head gaskets. NOTE: Head gaskets can be tricky devils when they are in the stage of going bad when heated up, but not blown enough yet to fail a cold engine test.
Another rare problem involves whether you ever had a catastrophic water pump failure? Sometimes when the water pump bearings fail, it allows the pump impeller to move far enough that it gouges out the surface of either the pump housing (easily fixed with a new pump) and/or the engine block (bad news!).
I would really need to see the history of the car and the list of parts replaced in an attempt to solve this problem, so that is as far as I can go with what I have. Also, I'm not clear on what exactly was the initial problem that caused you to go to a mechanic? Was it overheating? Or did overheating start after some other repair? Details Man, Details!!!
The lower hose is not an unusual problem, but is often forgotten.
When you get it resolved, please let us know what the problem was,
The 'Ole Factory Rep (Not Subaru)