When it comes to improving the stamina of your forearms, the only solution is to practice. It's the same deal as with running, or swimming; if you don't train to keep the muscles fit, they will get tired. To train yourself, play until your wrists get fatigued (I don't mean a little bit tired, I mean around the point where you feel the need to shake them out). Once they're fatigued, it's time to stop; if you keep going, you're opening yourself up playing sloppily and getting injured. If you take it slow, your stamina will increase gradually. If you hurt yourself, you'll have to stop to heal, which will set you back a ways.
As far as technique is concerned, when you're dealing with repetitive chords, do your best to separate your hands' up and down motion from their side-to-side motion. The up and down should be taken care of by your wrists alone, and the side-to-side should be taken care of by the rest of your arm. This does tire your wrist out more quickly, but it also makes it easier to play more precisely. In Erlkonig especially, make sure that while your hand must stay in the same general shape to play an octave, do NOT tense it. If you find yourself tensing up, it's time to slow down and practice playing without tensing. Again, tensing the hands leads to tired hands, which leads to sloppy playing and injury.
For the "oscillations" (I don't know the proper term, either :P), make sure that you're using more of a wrist rotation than an up and down motion. Again, make sure that you're not tensing your hand up. And again, your stamina will increase gradually, but it will increase.