Downshift thunk at 30 mph. Upshift little bumps at 40-45 mph

I have a 1994 Mercury Grand Marquis v-8, 90,000 miles. Car in perfect condition, babied and garaged, no repairs other than routine maintenance. 6 months ago I started getting a thunk when slowing down, always at exactly 30 mph, when it downshifts. Only happens sporadically. About 2 months ago, while upshifting 40-45 MPH started getting a couple of little bumps (feels like taking your foot off the gas and then pushing gas pedal again very quickly) Again, very sporadic. Does not matter if hot or cold engine. So far they've changed spark plug wiring and harness, new transmission oil and additive, new fuel filter. Two transmission specialists cannot reproduce it on test drive (not surprising as very intermittant), and they have no real suggestion to make. They say the transmission sounds and acts fine.
Any ideas?