I use tvtime and on some channels i have clear picture when on the most channels i have a blue screen (or white) while i hear the sound and i know that are tuned correctly. Only on some 3-4? channels i have picture on the others i have clear sound...

I use tvtime and on some channels i have clear picture when on the most channels i have a blue screen (or white) while i hear the sound and i know that are tuned correctly. Only on some 3-4? channels i have picture on the others i have clear sound but no picture only a white or blue screen !
I live in greece and have PAL BG.

Any help would be appreciated.
My tvtuner is Avermedia Volar hybrid HD and i have ubuntu 11.10 64bit.

HELP!