why does my car have rough accel. rough at constant speed but runs well when applying more throtle
Sounds to me like either a mass airflow sensor or throttle position sensor. It's having trouble telling the ECM the "position" of the throttle or the amount of air flowing through the throttle body at idle, so it's hunting, and has a rough idle. You may also need a tune-up.
What engine do you have?
agreed with littlehorn
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