Soulgirl

I just got my WRX STi2... I have been driving it fine for nearly a week but today something odd started occuring. After gear change and applying gas it goes and then just dies... but then picks up again. Took it out again this evening and my Husband nearly came to a stop before it hit in again and then went ok. He had to come down through the gears to 1st and it still wouldnt pull away at all (5 miles per hour). Then it just shot off again.

Is there a simple explaination? I have a warranty. Yes I use Shell Optimax.