Engine Light on, O/D flashing while driving and Transmission acting funny on my Ford Focus Wagon (Hatchback)?

I have a 2006 Ford Focus wagon (HatchBack) and it has 131,000 Kms on it. Since last few days the car is driving funny following are the issues I have noticed

1. The Engine light is on continuously
What I did: I disconnected the battery Negative terminal for 15 mins and placed it back and the problem was resolved.

2. The o/d off light comes on when I drive (It blinks in ORANGE color). I noticed this light comes on as soon as I start speeding above 50 km/hr. I also have seen the temperature for the car went high all the way from Min to almost middle of its reading. I'm not sure what is going wrong here.

3. Transmission: It is acting funny for last few days. The transmission slips a lot I think. On first gear I have put a lot gas to go to second gear. Sometimes it gives me a jerk while shifting gear. It just happens when I again stop at STOP sign and starts driving then I have to push gas paddle harder to make gear shift.

I'm not sure what is going wrong with my car. Is it transmission fluid or something bigger or just a solenoid? I will appreciate any thoughts coming from you guys.

So, you disconnected the battery because the light bothered you. The light was trying to tell you something is wrong. The O/D light comes on because O/D isn't working properly. Get a code reader and get the code or codes.

1. No, you didn't resolve the issue, you just deleted the code that was triggering the CEL.

2. Means that overdrive is not engaging.

3. Sounds like it needs a new tranny, or needs transmission fluid added As soon as possible if it is low.