I have a 2004 chevy impala and about a month or so ago, I was driving and noticed that when I put my foot on the gas, my car would not accelerate. Once my car made a complete stop, I put my car in drive and reverse and it wouldnt budge. A couple of hours later,I tried putting it in drive and reverse and i moved. I was to nervous to drive it anywhere and occasionally I will start up the car to see if will drive and go into reverse, which to my surprise it will. What could this be??

for a month I have not drove the vehicle, just checked to see if it would move foward and go into reverse