As mentioned, clean and secure all the battery connections. Also all the grounds from the battery to the body and block. And the jumper from body to block.

I suspect the starter solenoid. Just worked on one a little newer than yours, and the intermittent nature of the failure made it tough to find.

If your battery cables look the least bit distressed, replace both of them with high quality cables. They can fail internally and cause all sorts of problems. At 10 years of age, I would give them a looksee.