Whig-it

I have a 92 civic hb, w/B18b in it. right now it am having a starting issue, where the starter won't turn over. Now this doe snot happen al the time, sometimes the car starts just fine. i checked the voltage @ the starter, the black.white wire, and if i ground to the battery it's 12.4V or so, but if i ground to the starter, it starts @ 12.4V and drops, with-in a sec, top 6.5V or so. My first thought was the ground was bad @ the starter, but if i run a wire from the battery to the starter, postive, the car starts right up. All fuses and realys seem to look good. On random note, if i drive the car after it not starting ,by key, sometime when i stop it starts. thanks for any help