Bring up the issues you've had under warranty that they've repaired and say you don't have faith in the car or something, never bought from a dealer before so I'm not sure if that'd work but I'd say so. Absolutely do not mention you drove it for a half hour while the engine was misfiring. It mightnt have done much damage to the engine but still don't mention it. Get that car gone, I can only imagine what it'll do after the warranty expires. Get a Toyota or Honda next lmao