1999 Mercury Mystique not down shifting

I have a 1999 Mercury Mystique 4cyl. I came off the interstate and noticed that while braking, the car still had a lot of power. It would not down shift. I put it into neutral, just to come to a stop. I shut off the engine and restarted. The tachometer shot straight up to 4000 and the car shook hard. This repeated each time I restarted the car. I disconnected the battery for 1 minute, reattached it, and when I started the car, it started and idled normally. Staying in park, I pushed the accelerator down, revving the engine, it will approach and stay at 4000 RPM. There was no engine light displayed during incident. What might cause this problem?