Glad to hear you found out your problem.
Ever since I owned a drz400, I have been putting locktite on the front sprocket splines.
It's a bit of a biotch to get off but not a huge job in return for piece of mind.
I usually ride with the plastic cover removed to keep an eye on the sprocket.
"Kelsow's helmet is so tight, it cuts off the circulation to his personality."
Ride: 2010 BMW F800GS
Site: dualsport.ca and rideatlantic.ca