Same here. Ubuntu 15.04. Radeon HD 7770 1GB. CPU: AMD FX6100. I'm running the
amd64 Ubuntu release.

As I said in the other report. The last message I see is:
(WW) RADEON(0): radeon_dri2_flip_event_handler: Pageflip completion event has
impossible msc XXXXXXX < target_msc XXXXXX

no other messages found in any log. The driver is correctly installed. All
binary drivers were previously uninstalled like this:

1) sudo apt-get purge "fglrx*"

2) dpkg -l '*fglrx*'

3) dpkg --purge <all listed packages>

4) locate fglrx
Manually removed all existing references to the binary drivers.

5) sudo apt-get install --reinstall xserver-xorg-core
libgl1-mesa-glx:i386 libgl1-mesa-dri:i386 libgl1-mesa-glx:amd64
libgl1-mesa-dri:amd64

6) sudo dpkg-reconfigure xserver-xorg