Alignment would be my first instinct. Once you're sure everything is tight get a good alignment shop to look it over, and by good I mean find one that knows what's involved in making an IRS 7 series feel right. The Volvo Green Book for the IRS describes several adjustments which are not arbitrary but vary according to how an individual car feels. One section deals with this (p.13) "In case customer complains that the vehicle feels or behaves abnormally when driving, in spite of correctly adjusted camber and toe-in, toe-in variation should be checked." This refers to toe-in of the rear wheels under various loads (case in point, going over bumps), and the "Max. toe-in change per wheel must not exceed 0.05 degrees." Thus a difference of 1/20 of a degree is recognized by Volvo as causing handling problems, and is corrected using shims between the upper link & the wheel bearing housing. Hence the importance of having the alignment done by someone who really knows their trade.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf