I've been fighting this for about six months and finally decided that I didn't get my main clutch side seal seated on the crankshaft right last time I split the cases. On a 10 year old bike, a bad clutch side crankshaft seal can cause this because it sucks up tranmission fluid into your engine as you are running. Look for loss of transmission oil, but in my case, I change my transmission oil so often, I've never noticed it was low.
Wrong float height can cause this, but unless someone bent your float or it leaks, it usually doesn't change settings much. Weak spark can also cause this, but my experience is my spark is usually strong enough or non-existent.
Most times, it is jetting. Keeping on the throttle will help minimize the problem, but won't fix it.