After 10-15 minutes of playing League of Legends my screen flickers to black, then a gret screen for a couple seconds.  The game returns to normal then, save for a corrupted mouse pointer that is pixilated, a problem that remains when I exit to windows.  The problem can only be corrected if I enable mouse trails (which renders the pointer invisible in game) or by a complete reboot.  Just upgraded from NVIDIA to an ATI card.  Please help!  This is maddening!  I'm running an ATI HD6770 on an AMD Athlon II X4 450 Processor with 6 GB RAM.  Am thinking of dumping ATI for NVIDIA if I can't find a solution to the problem.  This is a freshly installed card with the latest 13.1 driver. 

Did you do a thorough clean out of the Nvidia drivers before installing the ATI/AMD drivers ? Switching brands was once regarded as cause for a clean reinstall of Windows, but just doing a good clean-out of the old Nvidia drivers should do these days. Worked for me.

Try uninstalling the ATI driver (Catalyst Install Manager), and any un-installed Nvidia graphics driver components, via Programs and Features, and after rebooting run Driver Sweeper (or Driver Fusion if you do not have the original version). Select both Nvidia and ATI graphics drivers from the list and let DS/DF clean the remaininf files. Reboot again and try reinstalling the 13.1 driver.

Which Nvida card did you upgrade from ?

Probably the solution is the same as it was years ago. Update the BIOS of your motherboard.

CPU: AMD Phenom II X4 810 @ 3250MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 850/1100MHz | Display: Chili Green Vision L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 95 x64 Edition

CPU: Pentium 4 Northwood S478 @ 3200MHz | RAM: 1,5GB DDR 400| MoBo: Gigabyte GA-8S661FXMP-RZ | GPU: Abit Geforce Ti 4200 | Display: Dimarson 19" CRT | PSU: Noname 400W | OS: MS Windows XP Pro, Fedora 18