I have 91 NSX 5 speed manual, 52,000 miles. Lately I've ground gears a few times downshifting from 5th to 4th. Wasn't doing anything fancy at the time. First time I thought maybe I had somehow screwed up. But a couple times since I was consciously pretty careful and still got the grinding. Doesn't happen every time; I've also made the shift OK a few times. What is probably going on and what is the prognosis?