lumpy running at 2500to 3500rpm

Hi I was wondering if anyone could help me. I bought an octavia vrs diesel at the weekend from a garage 300 miles away., driving home it was fine but noticed yesterday morning when it hut 2500 rpm it dies for a second, comes back to life then is really lumpy (if it was a petrol you would think it's miss firing) until about 3500-4000 rpm. It does this in all gears and it's doesn't matter if you stick the foot down or if you let it build up the revs gradually. As soon as yo in below 25. rpm it's fine again. I contacted the garage I bought it from and they're having nothing to do with is as they gave me a warranty policy I can use at any garage. I've got it booked into the Skoda dealer near me on monday but I was wondering if anyone knows what's up with it?