I got one of those RSM-4 mics a few months ago -- my ribbon was indeed sagging.
It was sort of tough to fix, since each end of ribbon was both clamped, and apparently glued to the ribbon motor, so I couldn't just pull one end of the ribbon and re-clamp. Instead, I had to bunch up the slack under one side and re-clamp real quick to keep the tension.
You can tell if it's sagging by looking at it - not sure which model you have, but most are pretty easy to take apart with regard to the grill. You do want to be really careful, since the ribbon is very thin and fragile, and if it breaks, it'll have to be replaced. You can pretty much take the mic apart enough to clearly see the ribbon without endangering it, though, I think.
I believe Michael Joly works on most of the Nady ribbons, for instance:
http://www.oktavamodshop.com/product_info.php?cPath=1_24&products_id=102
he does a number of things, including replacing the ribbon and the transformer, and he's done great work on another one of my (non-ribbon) mics, and has a bunch of other happy customers here. That's probably the best and safest way to go.