Yes it is alternated between the high-density routes of KEF
in the morning. Where it goes in the afternoon is more difficult to say. It has been used on high-density charters. The evening flights to LHR
are almost always operated by the 752s.
TF-FIX hasn't been photographed on a.net at any US airport so it's difficult to say if it can be seen there but most likely it's used to add capacity on highly booked flights to america if the need rises. Does anyone recall seeing one at, BOS
Hope this helps but I would love more info.