Raspberry loosing video signal sometimes

[namecaps]

Hi,

I have a weird problem which occurs occasionaly. Yesterday whilst watching Star Wars it happened twice. Raspberry is loosing video signal for a second, audio plays fine (I can see it looses signal as my TV shows blue screen which is a default screen when there is no signal). As I said it happened twice during one movie, sometimes it's not happening at all. Also i can see that there is a horizontal bar (brown or black) showing on the screen. Unfortunately I cannot make any screenshots because it happens randomly and dissapears too quickly.
I have a good HDMI cable (it was connected to ATV2 before). For the power source I tried on a generic 5V/2A (but good quality) and charger from HP Touchpad which is confirmed to work properly.
Any ideas? where to look for a solution?