Thanks G W-H,
My first thought was anti-ice but it didn't follow the usual pattern. (if Temp was +4 then icing would be expected from about 6000', so anti-ice would have been on continuously when on ILS, no?)
No tail wind component. it was a pretty stiff breeze. no turbulence reported, wind was pretty consistent from ground to 3000' which is unusual (no veering)
Update rate of the Mode S is between 4 and 6 seconds.
would IAS vary as you enter/leave cloud? It was BKN at 2500' and, I believe, overcast above.
Ps. all your assumptions are correct, except for the early go bit. what's one of those?