definitely a case for a professional repair; don't risk ruining the lens.Once I had trouble with a 90/2.8 beginning to produce slightly unsharp images after years of perfect performance. It turned out to a bit of dirt between the lens head and the focusing mount. This probably happened when the head was removed for lens cleaning. Your problem sounds worse. Perhaps a previous owner improperly reassembled the lens. Perhaps an element has become loose. Usually amateur attempts at repairs leave incriminating marks, always a danger signal.
Unless you bought it new, you never know what has been done to it (and by whom) in the past.
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