I think your spirited drive has given you a sign that radiator work is required, before a total failure. I had something similar happen with an early 140 series radiator, many years ago. If the neck itself is not cracked, then I think you can solder it back onto the radiator with a propane torch and some non-resin core solder.

You may actually see where the fluid is escaping, or an attempt by someone to do a temporary solder patch. You mentioned that the neck itself appears to be loose. Since the neck is made of brass, then it can be soldered back in place.

This job is best done with the radiator fully drained. You can do the job with the radiator in the car, if you are careful, or remove the radiator and also check the bottom hose connector, although this tends to be less of a problem. While holding the neck with a pair of pliers, heat the base of the neck and remove the neck - it looks like a hat that fits over a smaller neck that protrudes from the rad. Heat up both parts and remove all old solder with a wire brush. I think I used acid paste and the non-core solder to repair my rad, using a technique used for copper home pipes - capillary action. It lasted for the remaining life of the 145!

This is worth a try but if this repair is not successful, then you'll need to take it to a radiator shop for a more professional inspection and pressure test.

Good luck!
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Cam a.k.a. CVOLVO.COM