Try using the turbo button in game. There has to be something action on the screen for it to work, for some reason.
Describe "completely nuts" at high hz. Blurry text, flickering? Or worse? It could be your windows, or it could be a host of other things. Could be that your monitor is faulty, or a faulty display port cable, or that the problem is only temporary and will resolve itself. My Swift was unusable at 144hz for an hour (blurry text, minor flickering), and seems to be fine now.