Well that's some sketchy sh*t you've got going on there. One minute it won't crank with the key, next minute it will, and it won't start when cranking although you say you've got a spark at the plugs. (Do you mean that there's a spark present at the spark plug end of the wire, or at the spark plug gap?)
Also, a note on terminology. You say that it will sometimes "crank", but not "turn over". I've always taken these terms to be synonymous. What you describe as "turning over", I would call "starting".
I think your first step is to find out why your engine won't reliably "crank". Since at one point the starter wouldn't engage using the key but would engage when you jumped its posts, I'd look harder into the relay or ignition switch. For starters (no pun intended) see if you get voltage at (the 87 terminal IIRC?) when you energize the relay (turn the key).
Once you get the starter to work reliably, then you can make progress on why it won't start. Simple stuff first; no fuel/bad fuel (will it fire with starting fluid, then quit?), bad plugs, no/low fuel pressure ......