kendwood tv?

I have a freeview box attached but two stations keep pixiating when the weather is fine and sunny it seems the signal drops but when the weather is crap or cloudy the signal and picture are fine.

What type of booster do I need to 'kick ass' on the signal I can connect it in the attic on the cable run from the digital antenna. the cable goes to a four way splitter to feed two other tvs. I'm in UK taking singal from Isle of wight transmitter and live halfway up a hill so signal doesn't seem to be a obstruction problem or I wouldn't get pics at all.