Your help is needed

[Canadian Man]

Well this is just a guess but when your light's come on that is almost maxing out the 15a breaker. it's probably very close to tripping when the light's come on. after a few hours the wire start's to heat up considerably because it's having so much juice flow through it.

once the wire heat's up it provides more resistance in the wire and causes the load to exceed the 15a breaker.

Just my humble guess

I'm not an electrical expert and have no clue what I am talking about so take my opinion as very meger. maybe an electrician can pipe in and save the day