More Info ....
Highlight 'System' and select 'Properties' / 'Threads' are there are a number of threads listed with the same 'start address'.... ntkmlpa.exe!KeQuerySystemTime+DX50
The first four of these (and occasionally the fifth) are showing high values under CPU (totalling well over 90% all the time) and CSwitchDelta.
I suspect that this is the source of the problem - but have no idea what to do now.