hmm, there was something else wrong. Running at 80Hz wouldn't make you motion sick. Motion sickness happens for two reasons, people aren't used to the disconnect between what their eyes are telling them and what their body is doing. It takes a little while to get used to. The second reason is constantly dropping below the screens refresh rate.
The fact that you were getting headaches suggests to me that it was some problem with your IPD and the lenses. If I was on the old Rift and had the IPD set wrong, I would get headaches and nausea, just like you were experiencing. I have no problem with the Rift s though. For some people, due to head shape, IPD, plain bad luck, etc, just can't get the Rift S in the right position. It might be clear enough picture but, it's not quite right and that causes headaches. For those people, like yourself, there really needed to be a physical IPD adjustment like the old Rift.
No matter what the reason though, it really sucks for you.