2007 Renault Clio intermittent problems after starting when engine cold

My car is a R Clio Dynamique 1390cc 1st reg 1/4/07. Still under warranty.
I have an intermittent problem when i first start the car, that the garage say they cannot find. When i first start the engine everything is OK, when I rev up to 3000 revs its okay but once i engage gear and reach about 2000 revs it suddenly dies. I have to change up and down gear to try and keep the car moving. After a few minute it warms up and then works OK. There are NO warning lights. This has gone on for some time and I now notice that when the revs die if i put my foot down on the accelerator to try and 'push it' either nothing happens or it will suddenly surge back into life momentarily then die back down suddenly. Problem only in the first few minutes of starting when the engine is cold. Looking on the web i thought it could be something to do with an ECU failing to tell the choke to increase fuel when its first needed, i am no mechanic so thats just a guess.
First time in garage they did a diagnostic and said there was a recognised Clio problem and replaced the accelerator cable that was fouled up, BUT ITS JUST THE SAME.
I always thought it seems to be very lacking in power when driving, not what I would call nippy which i expected from a small 1.4 car but it seems to be worse than ever, quite flat. The garage also increased the idling speed, maybe to try and disguise the problem. I suspect they know where the problem lies but won't want to repair under warranty .