my engin starts to hesatate at around 30 mph and 40 mph. then stops hesatating t

There are a few possible causes for this condition. The most likely cause would be your Throttle Position Sensor (TPS). This is a electro-mechanical part which is subject to wear everytime the throttle is moved. The part of the sensor most vulnerable is 1/4 throttle, right about cruising speed. Try this firsr as it is easily replaced and not too expensive.