Mitsubishi FTO sluggish - poor performance

when applying a lot of acceleration the car struggles like it can't deal with the sudden demand for power. It splutters and can't seem to find the accelaration. Being an automatic it tends not to drop into first gear unless the car is completely stationary, if at rolling speed i.e when coming out of a junction it will pull away in third which I think is possibly where the problems lies but can't be sure. It also has the same spluttering problems when applying a fair degree of acceleration up a hill, the only way I can avoid it is to very gently increase acceleration but the car goes so slowly it is painful - any help would be much appreciated. Mr O