Strange problem when engine warm

Hi all,
I have bought my daughter a nice little 1.0e (2004 k12) to learn to drive in. She loves it.
The problem is after about an hours driving it loses all power. The engine will rev, but as soon as you try to pull away the revs drop and it really struggles. It feels like fuel starvation..
I have cleaned the throttle body and the sensor on top of the air box (maf/map??)
The car does not throw any codes, and if left for 10 minutes to cool down it will run ok for another 15 minutes or so before doing the same thing.
Has anyone had the same fault? Or any ideas what the problem could be.
Thanks, Dan.