Ok, mystery solved. Problem definitely was/is the Sony S-RGB transcoder. Basicly the svideo signal had too big voltage level for the Sony to handle, so it got all confused and lost the sync on all-white scenes.
I fixed it by adding some extra resistor on the luminance (Y) signal of the svideo cable to drop the voltage level. After that all-white scenes started working perfectly fine and the Sony transcoder doesn’t mess up the signal anymore!