If you Z offset does not remain constant, does that not indicate that you're missing steps on the Z axis? You've implied that you have multiple printers - does it happen with all of them? You say the problem tends to happen after a purge - can you give more detail about the circumstance? I'm assuming this is a purge at the start of a print. Can you make bed levelling and Z offset part of your print setup procedure? Does your Z axis suffer from overheating? Have you tried fettling the vref? Are your Z motors wired in series or parallel? Is it an issue with one or both of the Z axis stepper?

Sorry for all the questions, but it's an issue with many possible causes.