2004 Cavalier difficulty starting

I just recently bought a 2004 Cavalier and it hasnt given me much problems yet, but a few weeks ago it wouldnt start. So I popped the hood and checked the timing belt and I tried to find what was wrong with it. So I rocked the car back and forth and then it started. Then about 3 days later it did it again, so I solved the problem the same way. It is now doing this almost everyday, and I always have to rock the car back and forth just to start it. Does anyone have an idea as to what the problem is? is it the starter? the transmission?

Check the upper motor mount and see if its blown. My upper mount took a crap on me and it caused a world of hell. Car didnt want to start, tried to stall out every so often and I think I was getting like 200 miles on a full tank of gas.