Probs with tv screen bouncing when usi...

when i change channels or use menu on sky box the screen wobbles from side to side for a second or 2,ive had 2 brand new sky boxes and its still the same it only does this when switch to rgb scart though but when i use another scart or switch to pal on the sky box the picture is not as good,plus i have tried different scart leads and checked all other conections and leads so has the sky engineer and we cant find a problem can you help