My 49 Dodge behaves the same way, started about a year ago. I have been contemplating changing the fluid in the coupling but it doesn't leak so I really don't want to mess with it. Just been driving it like a normal 3 speed but sooner or later I want my FD back.
Have you checked the fluid level/condition? Could be like mud, who knows... Hmmm, I think I'll check mine this weekend
James Douglas has done extensive research and testing with the FD, he probably knows more about them than anyone else here.
I read somewhere thicker circulating oil makes it slip more??? I would think the opposite is true.
Thinking of filling mine with transmission fluid with NO seal additive and check, that would be about 7.5w. If I kill it, I've got a 440 sitting around, or just change it to a clutch and be done with it, who knows?
Sorry, just thinking out loud. When it was working normally, it sure made it fun to drive...