I have a 99 Honda Accord (4-cyl) with automatic transmission. It shifts correctly when it is cold but when it gets warmed up, the transmission jumps back and forth and when accelerating, it will not allow the engine to exceed 4000rpms. The check engine light is on.

A friend checked it and advised me it was the transmission but 1) why is the check engine light on and 2) what is it in the transmission that prevents the engine from exceeding 4000rpms when accelerating. (in cruise it will exceed 4000rpm but not when you push the accelerator to the floor)