Question for the SolidWorks flock regarding a new behaviour that is driving me slowly mad...

Background:

• I have a master model for a series of saved-out parts that is driven off a series of not very complex reference sketches.  These sketches all reference a single (simple, triangular) master sketch ...
• The model is a moulded enclosure for an electronic assembly and contains some mildly complicated surfacing and the usual collection of moulded in features.
• There is some referencing between bodies but I try to keep that to a minimum due to the pain of reworking client-requested changes which may or may not be drastic ...
• Model has 600-odd features and not even close to some of the horribly complex (and stable!) models I have worked on in recent years
• I'm running SW2018 Sp5, W10 b1809 on a Lenovo P52, 16GB ram, Quadro P2000 -  drivers up to date, including broken trackpoint firmware (...)

Problem:

• Reference sketches at the top of the model tree fail on rebuild, causing the feature tree to be daubed in blood'n'pus
• opening sketches to investigate reveals that spurious references have been added to external geometry (other reference sketches) - random coincident relations, incorrect lines referenced for parallelism etc.
• correct sketches, rebuild and carry on as before
• random time later after editing downstream feature, ref sketches fail and entire edifice comes crashing down...

I have gone as far as completely clearing and rebuilding all sketch relations from master and reference sketches but the problems keep occurring (never quite the same error, though).  I have seen errors toggle on rebuild on other models (fail/fine/fail/fine etc.) that I traced back to dodgy modelling and body errors but there is nothing obvious that I can see here!

Before I go and annoy my VAR, have any of you noticed this kind of behaviour before and if so, what was the cause?

Thanks!

p.s. sorry, can't share the model

## edit to note that this started two days ago ##