occasionally when driving car will not accellerate much past around 50 mph,

What is your question?

you can push pedal to floor and nothing. it seems to occur after driving several miles. other than that runs very well.You can stop,sometimes shut car down and then proceed and it doesnt do it again. would having two slightly different tires on front and rear be playing any part in this by affecting the traction control set up. Driving my daughter crazy!