When you say its a "pretty hard stop" can you elaborate more please? Is it more or less a "clunk" and stop or is it a slightly progressive "screech" and stop. What I am trying to get an idea of is how sudden the stop is - whether the focusing kind of stiffens even momentarily before it stops (screech) or if it just stops without warning (clunk). If the latter this kind of suggests something mechanical that has come loose and is impeding movement at a certain point. If the stop is less sudden, this could as others suggest (and you thought), be congealed grease though I am at a loss as to why this should work fine initially then lock up.I would expect the onset of that issue to be more gradual. In either event a service is in order.
I am put in mind of the fact that the Nikkor 24-70mm f2.8 AF lens (the original one from maybe 7-8 years back) has a fault where something would cause its zoom (not focus) to become very tight at some point. It was and is easily fixed by a CLA. In that case it was definitely a lube issue but its symptoms were more consistent with that diagnosis (a screech not a clunk if I can use that description once more). Point is such thing happen - especially to old lenses such as yours.