Computer keeps freezing up
Last week, I formatted my computer and installed Linux Mint 11. Previously, I had Ubuntu and had no major issues. Ever since I installed Mint, the computer occasionally freezes up and I have to power it down each time with the power button. Before it froze the first time, the screen turned all sorts of various colors and shapes. It was a big jumbled mess. It eventually went away but 5-10 minutes later the computer just froze. I believe I was using Google Chromium at the time. The second time it froze I had Chromium open but was navigating through the network places folder.
One time, it froze up when using Firefox. Since I also keep seeing weird scrambled colors on the screen, I wonder if it's a video issue. How would I troubleshoot this issue? I had Ubunu for months without any issues at all. Thanks in advance.