Hello!

Last week I bought a Radeon HD 6450 for use it with Ubuntu 12.04. The card was detected during the installation, but after reboot the system, I get a purple flashing screen instead the lightdm (but the intro-music plays). I tried solve this problem reconfiguring the graphics through the recovery mode and nothing works.
Today, mistakenly, I turned on my computer with the monitor unplugged (a LG Flatron E2060s), when I plugged to the card the problem with the flashing screen disappear. I can login and my graphics details say my system is using "Gallium 0.4 on AMD CAICOS".
However I reboot my PC and the problem is back. The only way I get works fine is unplugg/plug my screen.

How can I solve properly this issue?

P.s:
  1. I'm using a dual boot machine. The card and monitor works fine under Windows 7
  2. The graphic card is set "primary" on my BIOS.