I'd take it back to the dealer straight away - the longer you leave it the more they will fob it off. Having a stalling engine, while annoying, is also very dangerous.
They may have fiddled with something/knocked something or given it the latest
ECU update. Only they will know.
My car stalls when on constant throttle, usually in 5th at about 70kmh, has done since new, does it once or twice every 1000kms; dealer couldn't find anything so its just another annoying thing to get used to.