I'm getting the same problem with my AVR-635. It happens with both the actual video signal and with the OSD but if I turn off the video conversion in the menu then the actual video works fine. The OSD is still messed up with video conversion off. Even the blue screen generated when no input is present is messed up. This is all with s-video or composite inputs to an s-video output so conversion is not needed anyways. I too get several minutes of good output at power on and a power cycle gives another several minutes of good output. I believe I'm using the latest firmware. I tried a large fan on the video board keeping it very cool and it made no difference to how long the problem took to occur. The service manual has a block diagram and even a full schematic of the video board here
. With conversion turned off, the OSD chip LC74763M chip's output is going pretty directly to the monitor out so the problem is pretty localized.
Anyone found a solution to this?