I'm getting corruption in 1024x768 too. At first I just used the big fan on the top but that was not enough (its off in this picture). Instead I put a fan on the Indi, but that was not enough either, so I´m just testing with a big heatsink on the chips below.
That AND the fan on the Indi seems to solve it. If I remove either one, I still get corruptions.
This is not permanent. Just testing where cooling is needed, so its messy right now.