I have a MC2100-12A motor controller in this treadmill. The LED blinks quickly like communication is ok but it doesn't control right.The speed sensor also looks ok. When 1mph is selected it runs at 2.7mph and doesn't change as speed is steped up then suddenly will jump up to faster speed.

The speed sensor on these machines have nothing to do with controlling the speed. The machine will run without it. The sensor is obviously working, since you are getting a speed reading.
The calibration is a mechanical adjustment found on the motor controller, the instructions are found under the lid as well as the meaning of the LEDs flashes on the tech sheet..
Follow the calibration procedure. If the operation is still erratic, I would suspect a defective controller.
Calibration: Press stop and speed up. Install the safety key/magnet. release the buttons. Press stop 1 time. one window displays speed, the other the calibration number. Both should be "0".
Press speed up, the treadmill should begin to move around 25-30. increase to 85, the machine should be a max speed. adjust the pot on the motor board to max speed. Some machines are 10mph, others 12mph. The tech sheet will tell you the max speed. When adjusted, press speed down. the machine will stop. Now press incline up. That will calibrate the incline. When completed, remove the safety key.
If operation is still erratic, change the motor controller.. It could be the console, but the cost of replacement makes the repair in most cases uneconomical..