Intermittant Picture Breakup

Panasonic box (new from Sky under extended warranty) replaced the pace box due to blocky breaking pictures on some channels. Never the beeb or itv/4/5 some channels are fine others break up with a horible cracking squeaky noise. No trees in the way. the eng has called 3 times and this next visit, on Wed this week I am wondering what he can do. Some nights its ok and with the "same" weather the next night it will be bad.

Only thing left is the dish/LNB or cable. Both should be covered under the warranty(?) The lady from Sky said it was a rare problem (oh really?) but is becoming annoying after a couple of months of little or no problem. Any ideas?