Printer does not move in Z direction on large prints, suspect motherboard

Hi everybody, thought I would share a weird problem that has been happening with my printer.
I use the Sanguinololu controller obviously, and all is well - models are being printed as well as they can.
But! If I try printing something large - like [www.thingiverse.com] - a strange thing happens.
Printing starts as it should but the printer does not move in Z direction. LCD screen shows that it does (Z level count increases), Octoprint shows it does - but nope, it continues to scratch the bed glass layer after layer. And it only happens with largish prints. Firmware is Marlin nowadays but before I used Repetier and the result was the same. Any ideas/suggestions?

What does the G-code do? Are there actual Z movements sent?

What does the controller think where it is? Sending M114 should report what the G-code commands.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Well, what I have checked so far:
1) Z movement codes are present in the file just like in any other files - Z lifts, offsets, Z positionning at start of layer..
2) Octoprint shows Z layer changes corresponding to reality. I mean, when visually the printer has gone through 3 layers 0.2 high, Octoprint shows height 0.6
3) the LCD of Sanguinololu shows the same - all as it should be.
but!
The printer stays at level 0 mm...

OK, so you loose steps. Likely you can hear the motor beeping when it should move.

Things to check:

- Allowed maximum speed for this axis in your firmware, set it as low as 2 mm/s = 120 mm/min to be on the safe side.

- Allowed acceleration. Reduce to 500 mm/s₂ to be safe.

- Adjust motor current. ... the usual procedures, posted almost daily on this forum

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Generation 7 Electronics	Teacup Firmware	RepRap DIY