2006 Ninja 250 Engine cutting out?

When i start up my bike, it runs good around 3k rpm's and higher, but then when i let off the throttle, and head towards idle, the engine begins to putt and die out, eventually it just cuts out completely, starts right bike up fine, but then it does the same thing again. I just bought this bike, used, today, and it was running fine earlier. Any help?