Agree with the warm water routine. BTW it is the same for dairy cattle who drink gallons and gallons of water!
More water intake cannot hurt and I suspected insufficient water intake might be a culprit in repeat episodes if it is not sand, not lush pasture, tumors and the like. It is amazing how some horses (like some people.. ) just do not seem to drink enough water.
Good luck. Let us know what happens with the exam!