coolant container fills up

Yes this is likely with a head gasket failure (I had a green s70 glt and it blew 3 head gaskets in about 8 months before I lemonedthe car) and when the gasket blows, the system belches out almost all of the coolant in the reservoir.

If it is the head gasket failure, why is the car still drivable? It is also time to change the timing belt. What would it costs, to do both jobs?Is Is it worth it?

First check the coolant reservior, ensure no cracks. If thats good, then check the coolant lines, if both are all good then you have coolant evacuating somewhere else. If the head gasket is blown on the car, it could still run, but running would be temporary, and would be accompanied by smoking rough idle or the presence of oil mixing with the coolant (you didn't mention any of those symptoms, so blown head gasket is probably not likely). Since it is time to change the timing belt have your mechanic ensure that your water pump is functioning properly and NOT leaking (this could be the source of the coolant loss). Anyhow, good luck.

Thanks for everybody's help. I like the car, but with various causes I hate the thought of making heavy money into it.