Are the ingredient amounts different per serving? I think they all use eph and none use nor so the only main diff. would be amount per serving.
I know that ephedrine can have the opposite effect on some poeple making them feel sleepy after taking it instead.
Perhaps your getting a larger dose of ephedrine in the xenedrine and its over taxing your system so you body tells you it needs to rest in order to keep up.
Try cutting down on the dose of X and see if that helps