Two things... first, if you're actually drinking 96oz of water, that's gonna cause problems. Switch to a sports drink of some sort. I use gatorade mixed about 1/3 normal strength because it's cheap and easy to get but really any sports drink that tastes good is fine... they're pretty much all the same.

Second, how well ventilated is the room you train in? here's some food for thought.

Let's say that your average VO2 is 4L/min and your RER is .85. That would mean you're producing 3.4L of CO2 per minute. Times 90 minutes that's 306L of CO2 produced. In a sealed room that is 2mx3mx3m there is 18000L of air. 306/18000 is 1.7% CO2... by comparison air outside averages 0.038% CO2 and 5% CO2 is fatal. Breathing air that is 1.7% CO2 will probably give you a headache.

So.... open all the windows and doors you can, or get in the garage, or put the trainer outside on the porch.