Loading screen wrong resolution

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http://askubuntu.com – I've got a problem when booting: I see the GRUB screen where I can choose what exactly to boot into - after hitting Enter my screen simply shows me that purplish background and after some seconds it shows me the Ubuntu logo with the logo dots in a pretty low resolution (my guess would be 640x480); it then loads quickly and takes me to the login screen. So it's working, just not quite right I guess. My computer is a Lenovo Thinkpad x121e, with a AMD e350 CPU and an ATI Mobility Radeon HD 6310 graphics card (fully detected and working with the vesa:wrestler drivers). (HowTos)