If you are experiencing trouble at 86, you may want to have your Sonic checked out. I can lift my hands off the wheel and drive straight line at 85 for 15 seconds or longer, even following Tractor-trailers (which, actually, is why I did that experiment, too see why people were reporting major turbulence issues). Sticks to the ground in curves, drives steady and straight in the straightaways. No handling problems all the way up to 125, which is as high as I have gone... but I have gone 90 for hours at a time while driving across the country out west where the limits are higher.
There are better handling cars, no doubt about that, and you can improve the handling of your Sonic by doing a few mods... but straight out of the box, it should not be giving you any issues at 85, which is after all, highway limit speeds in some parts of the states.