90 should be plenty warm under the lamp. I usually start my chicks off on the cooler side, 85-90F. Is your temp 90 just under the heat lamp, or is the whole brooder that temperature? You might also pick up some Poly-vi-sol (without iron, in the baby section) or some Pedialyte or Gatorade to add some electrolytes to their water. Sugar water will perk them up as well.