Wierd Problem

I have a 85 300sd, and when I bought the car it ran great!
About 3 days later, I went out to start it, and it just "clicked"...after a few tries, it started right up. So I went and had the battery tested. But it is fine!

For 3 weeks, not a problem!...Always started right up.

Yesterday, I took my family for a ride. Stopped at a food store, and shut it off out front, 10 minutes later, tried to start it, and there it was again!

Just clicks...like the sound of a dead battery!...But after 5 minutes later, and trying 5 times or so, it fired right up, and restarted the rest of the day.

This is confusing,. because it does it so far apart, and hard to pinpoint it.

Is this the starter locking up?...I cannot figure what else would cause this....It is just a "click", like the battery would be dead, but I still have lights and horn, and good battery....Please Help!