Freightliner marker light issues

First try a new or known good light cord. If nothing changes I would then check the receptacle wiring (what the cord plugs into) on the tractor and make sure all the connections are clean and tight.

 

Does it have the relays behind drivers seat?
Maybe a weak relay that’s not holding contact with the truck running?

 

Sounds like a marker light relay in the circuit breaker / fuse box or the light switch on the dash is getting overloaded and intermittently cutting off. The switch may seem fine on the outside but the contacts inside go bad from years of heat and load on them. They can work fine until you add the extra load on them from the trailer lights then it makes them wig out.