Your freewheel probably just needs lubing. I haven't seen a NOS Suntour, but they probably just used a light oil. (Which has evaporated). You don't want grease in there, unless you're careful to keep it out of the pawls. (if the grease gets in the pawls, it can slow their engagement, potentially leading to skipping and poor engagement.) Usually, I dribble heavy gear oil into my freewheels. It gets were it needs to go, and sticks around a bit better than lighter oils.

It's also possible the bearing adjustment is simply too loose, but suspect it's just dry.