Are you using the choke as you should? You see, you should let the bike fully warm up before turning the choke off. Depending on your temp you may have to turn the choke all the way up and then turn the bike on. If doing so, stay near the bike for the first 30 seconds as the engine speed will greatly increase. At that point turn down the choke a bit, until the revs are at around 2,000 rpm or under. But don't turn it down too much or the engine will die. When a constant rpm is reached, let the thing warm up for a good 3 to 5 min. Turn off the choke and let it idle for a bit. It should now idle normally, at around 1,300rpm to 1,500. When you start riding, do some very moderately for the first couple of minutes. Idiots will give a cold engine full throttle the moment they leave their driveway, as they want to look cool. The only cool thing there is the engine, and a cool engine doesn't run well (or lasts).
Hope this helps. Let us know if it continues to happen after you try the above
2004 Suzuki RM250 race bike. Motocross rules!
2003 Kawi KLX400/DRZ400 semi road legal
2003 Yamaha BW50 - wife's scooter
1983 Kawi GPz305 - grandaddy Ninja -sold-
2009 Kawi N250 - the latest addition