This morning I finally decided to install the latest Mac OS X 10.7.3 update. Everything went better than expected! The only exception was when I connected my external display. The image kept flashing and I couldn't see anything. After some testing, I think I've found the solution:

1. Open System Preferences.
2. Click on Displays.
3. Show display in menu bar.
4. Click on the menu bar icon.
5. Check if the selected screen resolution for the external display is appearing twice on the list.
6. If it's there twice, select the other one. If it's not, I'm really sorry! :P
7. You can remove the icon from the menu bar now if you want.