Yikes!!
Another theory for the cauldron...
As you were flying along, you were caught in entrainment (air being pulled in from outside) of a CB or TCU-related updraught. However, this updraught was centered off to one side of your track, and as it moves you sideways relative to the DME, this causes the drop in DME-related groundspeed (?)
Other ice-related theories then fit in
As the air you are flying in is now moving in the same direction as yourself, your airspeed now decreases
then the updraughts, downdraughts somewhere else in this mystery-cloud
when you popped out the bottom, can you remember if you were still on track, or had it moved you off to one side?
Personally, and with hindsight, I think the rapid rise in airspeed after encountering the downside may have been due to chasing the instruments, although if my airspeed suddenly went to zero I'd poke the nose down as well!!
Good story, well organised thoughts.
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Confident, cocky, lazy, dead.