You're car wouldn't idle at 250-300. That bottom line is 500 so it's idleing about 600-750. You could have a clogged idle air control valve. When the car is cold and that is either bad or clogged it can either give you a really high idle or it can bounce up and down. The one long pause could be code 10 which is the intake air temperature sensor on your intake manifold. When mine was bad I just got really shitty gas mileage, no bad idleing.