Dear Aldo,
1. Try a different brand of BB cup to check and see if that helps. I know this might not be easy with an old bike that you want to keep "period".
2. Loctite makes a variety of excellent products that will help take up the "slop" in a loose-fitting threaded assembly. Just be careful not to use one that is TOO strong or you risk having terrible problems trying to disassemble your bb.
3. Have a framebuilder melt some brass in there and re-tap the threads. Not recommended if you want to preserve the finish as this will burn off the surrounding paint.
4. (my favorite) Try and track down a set of Campagnolo oversize bb cups (!). Yes, they did list them and make some. They have an "O" stamped in the cup after the thread spec (i.e. 36 X 24o). I think they were made in .5mm or 1.0 mm bigger diameter. Might be easier to find Jimmy Hoffa, Amelia Earhart, or Judge Crater than to track these down, but then again who knows what is out there until you ask.
Good luck and thanks for the Pic of the Day!
Mike Fabian
San Francisco