Check your battery cables from the terminal all the way to the starter. Also there is a wire from the fuse box, under the hood, to the battery. I had a terminal that was loose and corroded. Sounds like you are not getting enough amperage to the starter. Also check and clean your ground wires. Keep us posted.
1991 YJ 4.0L, K&N air filter, SS headers, 31X10.5, Congo Cage, etc...
When all hope is lost, and you have run out of options, you can always break out the "HOLY HAND GRENADE"!!!! It may not fix it, but you will feel a lot better.