Twice this has happened to me. The engine shudders while driving, especially when braking from a higher speed, such as coming off the expressway. Then the engine shudders violently and the rpm needle flies up and down. The brake pedal gets harder and harder to push until it can't be pushed at all, and the car then won't go. The engine doesn't shut off, but the gas pedal has no effect. I have had the car checked out each time- the first time they told me it was caused by oil leaking past the valve gasket into the spark plugs. Then two weeks later it happened again. This time they say that the Map sensor was bad and needed to be replaced. The car is running again, but it still feels shuddery and I'm terrified that it's going to stall out on me again at a time when I can't get out of traffic safely. What on earth is going on???