CJ5 will not start sometimes (won't even crank)

I just bought a 1970 CJ5 and it starts-up great most of the time. However, occasionally, I will turn the key and nothing happens. The engine won't turn over and I don't even hear a clicking noise. I turn the key and absolutely nothing happens. This has happened several times after I have driven somewhere (thus, stranding me) but it has also happened once when the engine was completely cold (on a really hot day). When the Jeep won't start, I can usually come back several hours later and the Jeep starts right-up like there is no problem at all. Any ideas?

Note: I looked under the hood and the Jeep appears to have a relatively new starter. I don't know when it was installed.