I have a 89 cherokee 2wd 4.0 Sometimes when you go and try to crank it, it will crank fine. Then today I drove to work, got off work came home, went to crank it back up. It would turn over but no crank.. I noticed in the past with both my jeeps the RPM needle will jump when it turns over. and when it doesnt crank the RPM needle doesnt even budge. The only way I can tell if its actually going to crank when im trying to start it, is if that needle moves. I dont know why but in the past its done it and I replaced a solenoid wire end that fixed it. Now its messing up again.

 

My 87 on occasion does that.
I unplug the CPS and plug it back it and it starts everytime after I do it.
I've replaced it not long ago, and not sure WTF is going on there but it works and its a minor annoyance.