Car starts most of the time but doesn't sometimes.?

I have a 97 maxima. At times when I try to start it it will do nothing. The dash lights, radio and everything works but the starter will not turn over. I have replaced the starter and ignition switch and cleaned all the ground wires but I still have the same intermittent problem. If I wait 15 or 20 minutes it will fire right up and, I know this sounds weird, if i roll it a few feet or rock it back and forth in park it seems to start. I am at a loss here. It might start 10 times in a row but it only takes one inopportune time to really be frustrating. Any help would be appreciated.