Your mention of this happening going above and below 50mph almost certainly suggests it’s related to the lockup torque converter in the trans, which kicks in and out almost exactly at that road speed. It should be an AW71L in your ‘91. The lockup function is hydraulic and completely local inside the trans. There should be a noticeable change in revs, dropping down to a fairly fixed rpm of about 2100 rpm at that road speed, akin to the feel of kicking in/out of overdrive, but not the kind of lurching you describe. A) Check your trans fluid level -there was a recent post here on doing that, you need the engine running and preferably warm. B) Give it a thorough drain and flush -again, a recent post on that here, likely in the same thread, see also the FAQ here. C) If after a few good trips things aren’t dramatically improved then you'll probably want to think about a blockage or, less likely, something broken like a spring in the valve body, perhaps wanting to visit a good local trans shop for further advice if you're lucky enough to still have such a specialty shop in your area, and failing that your favourite, trustworthy Volvo mechanic with experience in servicing high mileage RWD Volvos. As I recall, lockup torque first appeared in 1989 with the AW72L in the 740GLE 16-valve and AW70L in the 740T, which transitioned to the AW71L being used in the later 740s and 940s (turbo only I believe until 1994 when it became standard in the NA as well).I don’t believe it appeared in any of the 240s, but I may stand to be corrected on that.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now