Transmission goes into neutral

I'm driving down the road an my car is fine, then all of a sudden it will kick out of gear. Sometimes I give it a little gas and it will be fine. It causes my speedometer to go haywire too. I have to give it gas an let off a few times sometimes before it corrects. Sometimes I have go pull over completely an start my driving over from a parked position. I can't figure out what may cause this.