'91 190E jerks from 20 to 30 mph: What's up?

My 1991 190E 2.6 (96,000 mi) jerks slightly when going from 20 mph to 30 mph. The dealer service advisor says it's nothing but everyone I want to
sell it to tries to use this as a reason to knock down the price.
1) Is this a big deal?
2) Can I replace transmission fluid and make it a smoother shift to get these buyers off my back?
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