1997 Taurus shifting problem

Not that the transmission on this car has ever been spectacular, but this weekend it has really started acting up.
Auto shifting into 2nd and 3rd is accompanied with a lost of power, a
"clack-clack-clack" which seems to come from the front left wheel, followed by a "clunk!" as it seemingly jams into gear. Accelerating on the highway (past 70 or so) gives the same loss of power and "clack clack clack".
It only seems to occur when torqued - if I speed up slowly it seems to shift fairly smoothly, at least after it warms up. Oh, and the fluids are fine.
It has worsened noticeably in the last 2 days - any ideas/suggestions? What does it sound like and is it worth repairing?
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