Air con goes cold drivers side only.Renault LAguna Dynamique 2004?

2004 laguna air con goes cold driver side only when moving at 50mph + pasenger side ok.
When static no problem garage cant find fault is there a heat sensor that operates in the airflow that could be the cause?
Only way to change condition is to increase heat to max 26C on climate control then try and regulate until it goes cold again and really cold somtimes??