Any ideas? Have several friends experiencing this problem. Would love to find a fix for them that isn't just, "Go out and buy two 2080Tis," lol. Still have the 1080Ti, too, if there's any testing I can do that might help.
Do you have performance graphs that show what happens in-game? And does this stuttering occur when you're not streaming?
I would try going into the Nvidia Control Panel and setting your Performance Settings for Rocket League to prefer maximum performance. It's possible that since your GPU sat 50% usage, it wasn't reacting appropriately to spikes in demand.