I'm not sure if you've change news servers or not since you swapped ISP's - you don't mention it specifically so I'm assuming you haven't. Telkom (SAIX) only allow people using their network to access their news server (Same goes for IS/MWEB/Web Africa) so you wouldn't be able to connect to the SAIX news servers once you've swapped over to MWeb for your broadband connection.
If I'm not mistaken, you'll need to swap from using news.saix.net to news.mweb.co.za. If you're on an IS based connection you'd have to use news.is.co.za, and I can't remember the web africa news server off hand. For more info check out the awesome South African broadband forums on
www.mybroadband.co.za There's loads of people there who can help with specifics.
If you can list the news server's you're using that would help a lot too. (As far as I know MWeb do shape news server downloads a whole bunch, but that shouldn't stop you from connecting)
Hope that helps.