I'd pull the plugs and take a detailed look. It will tell you if it's running lean and also if the timing is off. I don't think I'd make a blanket statement that 40 total is too much.....if it likes it, it likes it. The ground strap will tell you how close you are.
As for jets it's just hard to say. I would jet up both ends for an experiment.
I don't think it's valve springs either...but it's possible. You could be getting some harmonics at that rpm or some valve float. If the loss of control is really bad, you will start to see some X patterns on the valve stems under the rocker tips.
Can you post a large high res pic of some spark plugs?