This is a known bug. One that has been mentioned several times on the Ubuntu and Linux Mint forums as far back as July this year, and maybe even earlier. Honestly I don’t remember this happening on my old desktop computer – Intel Core 2 CPU with a 250GT NVIDIA graphics card. Lately, I’ve been having these lock ups one too many times since I purchased my notebook barely a month ago – Intel i5 2410 with dual GPU (Intel HD Graphics and Radeon 6470M). On both desktop and notebook, I have Ubuntu 11.04 installed with the main difference being, aside from the hardware, that the notebook sports the 64-bit version to fully utilize the 8GB memory.