Why is my sentra revving up and down at high speed

This sounds like a transition problem and it is down shifting to a lower gear and kicking up the RPMs to maintain the same speed.

if its an auto then downshifting is probably what’s happening, most gearboxes have that difference in ratio. depending on where you are driving it could be normal for it to downshift like if your climbing up a gradient and the engine is struggling or if you are putting the accelerator more than around half way down the ecu will be programmed to downshift. If it is an auto and you have the option for a manual mode try using that and go out for a drive, if you select one gear that’s comfortable for 70mph and go on for a mile or so as long as you have it in the manual mode it shouldn’t change. If this fixes the problem for you then you have narrowed you issue down and can get someone who knows a bit about transmissions to take a look.

Everyone always jumps to the conclusion of trans problem. Read your owners manual and stop putting the cheap gas in it or you will be replacing a cvt. The owners manual will tell you that you can't use the cheap 87 gas or it will damage your vehicle. I don't know why they sell cheap car that requires the expensive gas. But you can't do it. Over time it will damage the cvt.