I have a 98 Volvo S70 T5 2.3L Turbo. I've replaced my thermostat, I know may head gasket is good because I've done the checks to see if its bad and the oil isn't Milkey, the cylinders when I replaced my spark plugs had no coolant in them. There's no leaks from the exhaust manifold, but yes I already bought a new to do this weekend just incase, and I know my water pump is good because I have to do a quick start a couple times when I filll my radiator because it's a closed system and I have to fill it through my reservoir. And also my engine is never super hot to the touch when this happens and it temp gauge never goes past normal range. And yes my electrical fan works. Great I might add. Always blowing hot air out the front of everything. Any clues im lost.
I haven't looked to see if your caps are different on your reservoir vs rad. I recently had a similar issue and it turned out the caps were on the wrong items. very silly thing, but caused me some confusion.
It could still be a bad head gasket that's failed between the cylinder and the cooling jacket and it's blowing compression gasses into the radiator. But an exhaust gas kit and test coolant for presence of exhaust gases. That will confirm or deny a head gasket leak.