Make sure you have a very good battery. All cables/ wires, and connections in good shape.


Or that the charging system ,is working correctly.


Kinda sounds like the battery is getting drawn down , and isnt sufficient, to operate starter.


When it messes up again , connect a known good battery ,and see if it engages.


If it does , then its either battery or charging system.


Thats my guess anyway.

Good Luck,

KYhunter

Oh, I forgot, DUH, you can take starter to auto parts, and have it tested. Still check the batt. and charging system.