this is something that has puzzled me as well. i had a 2.6 installed and reflashed with a programmer. when i reinstalled there was no video.
try using S-video or connect it to an amp (using monster a/v cables) - those are the only 2 combinations i know of that will work.
i'm not sure exactly why this is caused, but someone has to know. thought about reflashing eeprom and thinkin it might help, but i dunno.