Bubbles in coolant reservoir

Unfortunately, this is usually a sign of a blown head gasket.

Check the oil for coolant contamination. Check the coolant for oil contamination.

Another test that you can do is a compression test for tall cylinders.

Depending on the age of the car, you may want to consider a head gasket sealer but keep in mind it doesn't always work.

You mentioned a new thermostat and radiator were installed. Air can get trapped in newer engines during such repairs and unless purged (usually in a vehicle specific manner), can cause a number of odd symptoms.

That's the easy one...

The bubbles and overheating at idle do also point to a head gasket leak or a crack in the cylinder head. If the old radiator failed and the engine overheated significantly, its much more likely to be serious.

Easy checks...

Check for a milky deposit on the underside of the oil fill cap and discolored antifreeze (assuming you have fresh coolant). If your engine is pushing exhaust gasses into the cooling system while running (the bubbles in the overflow tank), some coolant may going the other way into the combustion chamber when the engine is off. If so, look for white smoke (not the light, wispy normal kind) and a somewhat rough idle upon starting up after sitting overnight. Coolant has a very distinct odor when burned too, your nose...knows.

I have seen success with stop-leak products, if you choose not to fix the head or head gasket.

You will be on borrowed time...

It sounds like the head gasket. I currently drive an Opel Astra and have the same problem. The mechanic told me I’ve a few days / weeks with my car before it fully goes. But I’ve been driving the car months now. I can’t be kept in heavy traffic. And at traffic lights I switch off my car. I keep the car coolant topped up fully at all times. You can keep driving the car but won’t survive in heavy traffic. My coolant looks brown now. All the signs are there.

I defer to the previous answers but it is such a 'car-specific' issue as in the case that the head gasket is not blown nor is the block cracked and there are no compression issues after testing then it could simply be trapped air in the cooling system. Porsche water cooled rear-enders are especially suspect of this problem as well as some similar exotics which have large cooling systems with lots of bends and tubes at different heights. But bottom line is that IF your car need more and more coolant then something is wrong. You might double check that the repair put on the correct coolant cap. I've seen a car vomit hot water before after a repair only to find that the mechanic when cheap and bought a generic radiator cap that did not have the right seating design nor spring pressure to hold the hot coolant of the properly operating engine.