@Ramez I had that problem early on too. Try using a shorter USB cable, and preferably one rated as a "quick charge" cable that has larger gauge wires that will result in less of a voltage drop. A 2.5A supply would be ideal, but if you aren't powering much else off the USB on the pi your 2A supply should do. Many overclock scripts also include a voltage boost to the chips, so you want to make sure your power supply and cable can keep up too.