hi mariog7,
A couple things to check:
1) after you restart your Blackberry return to Options->TCP and make sure you new settings are still there (I.e.- they were properly saved)
2) call Cingular and verify that, in addition to a wap.cingular APN existing on your account, you also have the "WIX" based plan info, which will ensure you have full data services capabilities.
see
this post by Mark Rejhon
From your post it sounds like some data services are functioning (your reference to having problems with "certain" websites via the Cingular browser). Can you verify that you are able to access some sites, but not others?
If so, you may try explaining to Cingular that you are unable to use non web-based data services, and ask them to verify that your account is enabled for them.
Ryan