I had a few problems when I first got my X Flow two years ago.
I have done the following:-
Fitted new hoses - insrance really
Took the rad out at the same time and flushed it through several times.
Bought new thermostat and new switch for the fan
On mine the fan was mounted to far back so I bought and fitted a new fan that mounts direct onto the rad. I needed to move the rad fowards to do this. Be careful if you do this the clearance between rad and nose cone is small, it will rub through the rad very quickly.
I have also fitted an overide switch for the fan.
No problems now, even survived some log traffic jams en route to Le Mans in 05 in high temps. All of this is with a standard rad and it us a slightly uprated supersprint engine.