Starting issues....

Hey everyone so im new here and just bought a blazer 3 ays ago, now the thing runsmint and interior is really nice. My question is this, it will start fine some days. It wont hesitate just turn over and go. But once in a while it wont start...at all you will a click (either the solonoid or starter) and thats it. Then if you leave it for 5 mins it still start up as if nothing happened....anyone know what it might be? Im thinking that the starter is just stick cuz its an old truck. its got 340xxx KMs on it. Any help is appreciated.

My guess would be the starter too, or the relay. Since you can hear a click I would say the relay is good.