Slick 50 Octane boost for high mileage cars

[yankeefaninseattle]

Please offer some advice!! My beater is 1983 DL hatchback w/ 228k miles on her. She's been a great girl. I give her tuneups, oil change, plugs, fuel filter etc. Just changed the alternator, blablabla. So, I warm her up for 10 mins in the AM, leaving her running, and she would cut off. This happened for about a week, thought I'd try this slick 50, and now she won't stop racing. I checked the flapper in the carb, and it moved, so my assumption that the carb was stuck was wrong. I filled her up w/ 9+ gal of gas, on advice I needed to dilute the octane in the gas tank, this didn't work, the motor still chokes when I turn the car off, so I have to pop the clutch to get her to stop. When I am at an idle, the only way the revving slows down is if I have the car in 1st gear, press the clutch AND the brake until I hear the revving die down. How much more info do you need to give me some options to try?? BTW, the slick 50 was a fuel cleaner for higher mileage cars, octane boost was on the label. Please Help. Thanks. B.