The -ati package depends on the -radeon package, so if you remove -radeon both are removed. If you then install -radeon, there is no reverse dependency to bring in -ati.
It seems indeed that you have the same issue as Bastian in bug 650425, there's a broken 1360x768 mode detected when using KMS. -- [KMS] White Screen instead of desktop after 10.10 update (ATI Mobility Radeon 7500 card) https://bugs.launchpad.net/bugs/655099 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs