Problem with ford ka

I've just bought a ford ka, its brand new, 53 plate, and is only 3 months old. I recently filled it up with the usual unleaded petrol, but since then
everytime i put my foot down on the accelerator to pick up speed in certain gears, for eg 3rd/4th it chugs as though the petrol isn't getting thru.
I thought it could be some naff petrol, so i've ran it down, then filled it up with a tenners worth, just to be on the safe side. Its stopped doing it as much, but it still does it.
Does anyone have any idea what this could be? I know nothing about motors so i'm looking for people with experience.
If anyone can help, please let me know, before i take it back to the dealers to get them to look at it...
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