I have always found the Altea to be a tad twitchy on the motorways.
Try dropping a few KGs of weight into the boot just to rule out the suspension play. I did a trip in the first Altea to Kent - using the M6 toll road. It was late at night, and very quiet - but the Altea -due to having full detailing kit in it was solid.
I went to the NEC this weekend in the new one - un laden, and had the twitchy feeling due to no weight.
In my opinion, due to the height the suspension is slightly to stiff to prevent body roll, and this then translates down through and across the chassis creating the twitchy feel you are getting.
Just my thoughts.