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I don't think that's a pressurized reservoir. I think it's more likely that it has a radiator cap with a valve that runs to the overflow tank.

If it is overheating when idling and not overheating when it's driving down the road then it distinctly points to a problem with the fan. I know you say the fans are working "correctly" but you should really verify that in absolute terms since this is precisely the symptom of a fan that's not working.

The fan should have a thermostat that turns it on and off, one for the A/C fan and one for the radiator fan. They should both blow in the same direction, and that is they should draw air through the rad into the engine bay from outside. You may also want to check and be sure the radiator fins are not bent or smashed near the fan, or that there is no junk in there so that the only radiator fins that are not obstructed are the ones outside of the fan area. However I think since you said you replaced the radiator that the odds are relatively high that the fan(s) are not hooked up or working correctly. Maybe the thermoswitch for the fan is in the wrong place, maybe the A/C and rad fan thermoswitches are mixed up, or some other problem. Get a shop manual and go through the troubleshooting in it.

I hate dealing with cooling system problems. Two H2O VWs and a 70s BMW did me in. My Miata has a 5-core aluminum racing radiator. Now that's cooling capacity!

EDIT: oops I missed page 2 of this thread, looks like you got it fixed. But I would caution you against buying a VW if you don't like dealing with cooling system issues. I had two VWs that both were plagued with problems. Basically the problem is that they use a couple of plastic fittings in places where a Real Car would have metal cast parts. These can crack or expand and deform under heat and cause the pressure to drop and coolant to blow out as steam, which drops the boiling temp and the car will overheat every time. By the time you turn it off and get the coolant refilled it has cooled enough that the leak goes away and you can't find it. I did this over and over with these two VWs. Once you know the fix it is easy and you just have to keep an eye on it. But I'm not convinced that in 30 years VW has really figured out how to do water-cooled yet.