Peterbilt 387 ,04 starter relay

Thanks in advance for any help.
Ok sometimes in the morning when I'm getting ready to star my day.
I turn the key on and it starts and starts but it never get to crank until I replace the starter relay then it crank I go on. Then 1 week and sometimes 3 days later I go true the same problem. It use to happen every 5-6 months but now is getting really bad.
Sometimes it turns off while am driving I stop change starter relay and keep going
(Relay in passenger side next to the coolant container)out side Thanks again