On out 740s the following has helped -

First try, add a washer under the nut - this forces the arm further down on the tapered shaft. Don't forget to clean the splines. Some people have had luck with locktite.

Second fix, add material to tapered shaft. A thin piece of soft metal can make the fit tighter. I have wrapped fine copper wire around the taper and this has worked for a year or so.

Third fix, look, the hole in the arm is now huge - you need a new arm!
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96 855R, 95 855,854, 90 744 Ti - 340,000 Volvo miles put on 7 bricks