Software Update dodgy since upgrade

Since upgrading from Jaguar to Leopard, I've been having no end of problems with Software Update. I installed to a clean drive from a 10.5.1 installation disk, but it gave old versions of iTunes and Safari, and Software Update showed that everything was current, even though 10.5.5 was available. I had to download all of those manually. After that, I got one Software Update list that was fairly long, and installed everything properly.

But since then, it's gone wonky again. It takes about 10 minutes to figure out what updates are available, and then it refuses to download them, claiming that it can't find the update server and to make sure I'm connected to the internet. My attempt tonight didn't even show Safari 3.2.

I am connected to the internet, surfing everywhere I please no problem. System Preferences diagnostics says there's nothing amiss. Any ideas why I'm experiencing this?

 

Oops, I meant since upgrading from Tiger.

Anyone?