Re: Angina threshold - why does it change?
My thought is that you have ingested something that is altering the vasoelastic properties of your arteries, and that it is acting differently day to day when you exercise. You say your diet is steady and you "mix in a few herbs"....well, those details might be at play here. Various supplements act as vasodialators, I know because I am taking them all....fish oil, magnesium sulfate, bromelein. Others may not dilate but may relax arterial walls. Maybe a more careful logging of intakes and correlate with the differences you see in exercise would tell you something.
Just guessing here....