99 protege doesnt want to start when cold

[BrOkEcAr!]

We just recently bought my car used and its a 99 protege. Everything works fine but i noticed when it is less than 50 degrees outside, i put the key in and it cranks okay. As soon as i start going it cuts off and wont go anywhere. It works fine in the middle of the day when its warmer. yesterday, We took it to a mechanic and they tested the engine for different problems, and they drove it and said there was no problem. Then this morning it would crank up but as soon as i put it in drive and let off the break it dies and wont do anything. I have no clue whats wrong with it so pleasssee help me!