Display keeps blacking out (i915)

I am experiencing a very annoying display problem on my laptop, which I am struggling to troubleshoot: The display is frequently blacking out. When it does this, the system keeps running in the background (not a complete system freeze); however, the display stays off and the only thing it seems to respond to is if I switch virtual terminals (with CTRL-ALT-F1 etc). When I do that, the display comes back on again, but usually blacks out again within a few seconds. The blacking out seems to happen mostly when I am in an X graphical session, and seems to coincide with events that cause fairly significant refreshing/redrawing of the screen (such as if I open a new application window or new web page). If I am working in a console terminal, it seems not to happen (or at least, not as frequently). I am running Trisquel 8 (based on Ubuntu) on a Macbook 2,1 (it is a fairly old machine). The kernel version is 3.13.0-135. I have tried a slightly older kernel (3.13.0-39), but experienced the same problem. So, I am obviously trying to figure out what is causing the issue, and whether it is due to hardware or software. The problem started about a week ago - before that, the display was working fine. Around that time, I updated a few packages and there was probably an initrd update, but I didn't update the kernel or modules. **Edit:** I tried booting up the machine from a Live USB (both Trisquel and Lubuntu). Neither of them worked. When I tried to boot Trisquel, I got a printed error message saying i915: Invalid ROM contents, then the screen went completely black. However, the system carried on booting in the background, because I was able to blindly open a terminal and type reboot to reboot the system.