Check engine & AT light

I have a Mazda 3 2004 sedan. It has 4 speeds. It has about 125,000 KM. I get all the normal oil changes done and brakes as these are normal. I noticed about a year back, from time to time the check engine light and the AT light would turn on. When the car is in park, and you move it to reverse it jerks. It shakes the whole car. It would turn on and then it would just disappear on its own. I didn’t think it was such a big deal. Then it came on again, and I went to a garage to get it checked out. He said that my transmission was done, and quoted me $900 for a rebuilt transmission. I just said ok and left. The lights disspaeared and took it to another mechanic. He told me the car is fine. It runs like a beauty and couldn’t find anything wrong with it. The lights continued to come on and off. For the past 2 days the check engine light and the AT light have been on. They haven’t disappeared. The car jerks from parking to reverse. It slightly jerks while shifting from time to time. I need to know whats wrong so I can get this fixed before it gets worse. I read some forums and they recommended doing a transmission fluid flush and someone else said, “The "Input Turbine Speed Sensor" is also called the "Pulse Generator" and is on the top of the tranny near the front of the vehicle and close to the engine. The part number is FN01-21-550. If you are getting a P0715 code, this is the one to change.” I need some advice please.