Please Read [url[http://www.samsunggalaxysforum.com/samsung-galaxy-s-updates/if-you-are-facing-problems-after-upgrade/]this[/url].
Thanks for the info, IronSingh. This info is good advice and being a Computer Engineer, this is also a good 'rule of thumb' to follow. :-)
I've done both an upgrade and a fresh clean install, and the phone behaves the same in both instances as soon as I turn on the WiFi. The ROMs I used were BUJV9, XXJV0 and XWJVH. I was forced to return to BUJV3, Build XWJVB.
This problem will not be mine for much longer anyway as I'm going to sell my Galaxy S and buy the Galaxy S II which comes with the latest version of Gingerbread. Of course I'll be upgrading to 2.4 once it comes out. :-)