From what I have seen over the years, a converter that has lost the clutch will generally chew itelf up in fairly short order and things will start failing to work right real quick.

Checking the pan for debris will tell the story.

I think the coolant temp anomaly is just about what you are thinking, unless there is a gauge issue creeping into the picture.

Swapping out the sending unit would possibly give a clue along these lines.

Missy