Should it be expected that some shielded wire is not good at data transmission?
Interestingly I was replacing some solid core ethernet cable I'd
  hacked in to do the job, moving up to a wire type I assumed would be
  far superior. Turned out it wasn't to my surprise.


Cable that has a characteristic impedance of 50 ohms tends to fall into the 100 pF per metre region and if you look at the formula for impedance above several hundreds of kHz it is approximately this: -

\$\sqrt{\dfrac{L}{C}}\$

This means that the inductance is about 250 nH per metre (with a touch of hand waving). For the impedance to double to 100 ohms, something has to give and clearly the ratio L/C has to increase 4 times. This usually (and again with a bit of hand waving) means L doubles and C halves.


  I have dropped it from 400 kHz to 200 kHz as I can see the waveforms
  are just about good enough to get the job done. It works for now.


This might easily be enough of a reason for things not to work at 400 kHz clock but become workable at 200 kHz.