Renault Modus jerking on acceleration

My daughter's Renault Modus 1.4 suddenly developed a problem - it jerks when accelerating, like a lean surge. It is fine when cold, for about 1 km, then it starts.
I suspect the coolant temperature sensor. Can anyone tell me wher to locate it and what tests can be done with a multimeter?
What else could be the problem? :screwy:

the problem is that is a renault's...change the car because you will go from a problem to an other one...