[IDave]

Can't say I really know, but a quick google around the net suggests this isn't exactly a rare complaint. I agree with those who suggest that its related to the relative concentration of some apple component, presumably acid content or fructose, or perhaps both together.

I might have missed whether this is a relatively new problem or not, but a thought I had is this: perhaps you have an ulcer or gastritis recently, and a bolus of apple juice on an empty stomach is irritating (you said that it's relatively dose dependent: 10 oz is worse than 3 oz). If you drink juice after a heavy meal does it act the same?