My car doesnt feel right on start up ?

[Bilzaaa]

Last week, when i turn the engine on and put it in first gear then move off, it studders if you get what i mean? Feels like its got power, then loses power then power, then no power that makes the car shares? Then when thats finished, i go into second gear, about 12-15 mph, it does the same but this time i car hear blowing noses from either the egine or exhaust but i think its the engine. When driving the car for a while, this problem wont happen, eg at traffic lights. It only happens when i leave the car parked for about a couple of hours then i go back out and the issue happens again.

What could be the problem? I phone the garage and booked it in to be checked and hopefully under warranty. Btw its a Fiesta Zetec S