  We continue the analysis of NuSTAR data from the recent discovery outburst of
MAXI J1820+070 (optical counterpart ASASSN-18ey), focussing on an observation
including unusual flaring behaviour during the hard to soft state transition,
which is a short phase of outbursts and so comparatively rarely observed. Two
plateaus in flux are separated by a variable interval lasting ~10 ks, which
shows dipping then flaring stages. The variability is strongest (with
fractional variability up to $F_{\rm Var}\sim10\%$) at high energies and
reduces as the contribution from disc emission becomes stronger. Flux-resolved
spectra show that the variability is primarily due to the power law flux
changing. We also find a long soft lag of the thermal behind the power law
emission, which is $20_{-1.2}^{+1.6}$ s during the flaring phase. The lag
during the dipping stage has a different lag-energy spectrum, which may be due
to a wave passing outwards through the disc. Time resolved spectral fitting
suggests that the lag during the flaring stage may be due to the disc
re-filling after being disrupted to produce the power law flare, perhaps
related to the system settling after the jet ejection which occurred around 1
day before. The timescales of these phenomena imply a low viscosity parameter,
$\alpha\sim10^{-3}$, for the inner region of the disc.
