Possibility:
Revving the piss out of it when first starting up before the engine has a chance to properly warm up and the oil to circulate. It has nothing to do with cam chain wear. As was stated, if it were related to the cam chain, you would have bent valves.
If I were you. I would pull the cams and check them for scoring or wear along the bearing surfaces. I would also pull the clutch cover and take apart the oil pump and check the lobes for wear. And, when you do that, pull the oil pump screen and look for debris. You might be surprised at what you find.
