Car hesitates but no check engine light on

What is your question?

The last 4 months my car has been hesitating with check engine light on.I took it to Auto Zone and 2 codes came up to replace the air intake sensor and knock sensor. Now it doesn't happen consistently,some days are worse then others whether the car is warm or cold.Between 1500-1800 rpms the car starts chugging after 50 mph it smoothes itself out. Replaced the air intake sensor at a mechanics garage but not the knock sensor, and it is still doing the same thing but check engine light is off now. Mechanics I have spoke with said there is nothing they can do because the engine light is off. They said if its your fuel injectors you won't get a check engine light. What could be causing this?