A month ago, after the most recent update to OS X (10.8.2), we ran into an issue that caused certain Macs to restart when using Pixelmator intensively. Sometimes the application could stop responding without having to restart the computer or just cause some graphics crashes.
We did everything to find the root cause of the problem. We researched a vast amount of information about the inner workings of OS X. We demolished our Macs. We even ask for Apple's help. Finally, we found out: an issue with NVIDIA's GeForce graphics drivers.
They found the cause of the problem, but not the solution. Meanwhile, they continue to work to resolve the bug even with the help of Apple. The affected graphics cards are: GT 120, GT 320M, GT 330M, 9400M and GT 9600. With or without solution, the Pixelmator 2.1.4 should arrive soon, bringing many improvements. (MacRumors)