OK, now we're getting more information. The chain jumping around at high speed pretty much says you need a new one, but to be sure of what's happening, put the bike on its center stand and rotate the rear wheel, observing the chain movement, how much slack there is, etc.. If it's anything but smooth movement and if even one link is stiff, get a new chain. Inspect both sprockets too, because especially when a bad chain has been running on them, they're likely to need replacing at the same time.
If the chain is causing the problem, it's a dangerous condition. A broken chain can cause a crash. Don't wait, check it before riding any more.
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