The actual reason of the unstable ppm server was circular dependency of Test and PathTools, though they are both in the core and I should have ignored them. Now I added some codes to see if the dependent modules are in the core of 5.8.1 (a bit modified, though), and if that's true, omit them. Sorry for 5.8.0 users (if any).
I haven't written extra codes to check other circular dependencies, though I successfully installed latest Plagger/Jifty/Catalyst/DBIC from there. If you find anything, let me know.