Ubuntu 11.04 Reboots & screen flickers

Question

I'm a newbie with Ubuntu 11.04. My Dell Vostro recently have been giving me issues. On boot my pc screen shows lines & sometimes the screen flickers similaly like a tv screen with bad signal. When the screen flickers the pc does a reboot without me doing anything. I did a manuall power down a couple of times and then pc boots up normally without any issues. Can someone advise how i can resolve this.