If the guy has been in business for >20 years, you'll hardly impress him with yelling at him.
Like others have said, your goal here is to get out of this car. Go there and tell him just that. Tell him you need a reliable car and this will never be it. It has been monkeyed with and who knows what it will take to get it to run right. Ask for a refund first. If there is no budge, ask for a refund minus a loss on your end and take the loss.
No way in hell would I try to get into another used car from this guy.
Lesson learned is not to buy from a used car dealer. You have to find such a car directly from grandma or the estate. Especially on a run off the mill vehicle, there is NO reason to go through a dealer. It only adds a bunch of armor all on the car and risk of shenanigans like this. Sounds like an impulse buy - never a good idea. But I am sure you know all this by now.
Good Luck!
G