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Since it is an older lens, I'd recommend that you have the lens cleaned and calibrated. After a while the lubricants tend to get sluggish as well as some dirt buildup. I sent my older 300mm f2.8 to Midstate camera in Warwick Rhode Island. They cleaned the internal optics, re-lubed and calibrated the lens for less than $200. It came back working like new. Turn around time was less than two weeks.
Good Advice, if it has not been recently serviced by a third party company, do it. If it needs parts, its better to find out ASAP, they might still have some around.