This pretty much sounds like your dish/LNB is not aligned correctly for the Greek channels. I doubt it is a bad tuner, LNB or power supply. Try deflecting the dish a VERY little at a time in all directions while watching (or get someone) the picture, or even better the SNR while you do this. I bet you will find that when you push it in a certain direction, the picture will improve. these dishes will typically take a reasonable amount of flex without breaking them, so just flex it without loosening the mount, at first.
IMPORTANT: You said you replaced the LNB? If this is the case it might not be the dish is out of alignment, but the LNB might be out of focus. Loosen the clamp that holds the LNB and move it backwards(away from the dish) and then forwards(towards the dish) until you get the best picture. Then tilt it from side to side until you get the best SNR.
LNBs are essentially 4 dimensional:
a. up/down b. left/right(important if you have a multi-lnb bracket or dish has been moved), c in/out from dish(focus the signal) and last but not least twist LNB(clockwise and counter-clockwise) in the bracket.
The last aligns the shape of the receiving antenna with the shape of the reflected pattern from the dish and improves signal quality as it also improves seperation of the Vertical and Horisontal signals. If it is stuck somewhere in the middle you will get a lot of noise.
I hope this helps.