SOMETIMES when I try to take off my car will accelerate but won't move

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THE PROBLEM JUST STARTED AND IT DONT DO IT THAT OFTEN BUT OFTEN ENOUGH EVEN WHILE IM DRIVING. I CAN HAVE MY FOOT ON THE ACCELERATOR AND MY RPM MOVES UP BUT MY CAR IS IDLE. AGAIN EVEN WHILE IM DRIVING AND WHEN IM TRYING TO TAKE OFF. BUT IF IM SITTING STILL AND IT DO IT ALL OF A SUDDEN IT TAKES OFF