Z step rate issue

Hi all, can anyone help me on an a problem I have with the Z axis?
I have an i3 prusa, everything seems to work correctly when the Z axis step rate is at default (M92) somewhere around 500 or 600 steps per unit if I remember. However at this setting a 20mm x 20mm x 20mm stl box gets printed at X20mm Y20mm and Z3mm, basically it becomes a messy blob, I had to set the Z axis to 4035 steps per unit to get a fairly decent 20 x 20 x 20 printed box, the problem is however the z motors occasionally stall and are fairly noisy at this setting, I have also adjusted the stepper driver but this does not seem to help at this step rate.
Is setting the Z step rate to 4035 to high and should I change other settings to get the correct calibration?

I also seem to have a similar problem with all my motors except it is the other way around(way less steps permm than normal).

Sage

Setting steps/mm isn't a matter of playing Bingo, it can be calculated precisely. For a threaded rod axis it's

(steps motor per turn) / (pitch of the thread)

Half-stepping doubles the number, quarter stepping requires * 4, etc. M8 rods have a pitch of 1.25mm. If this doesn't work out, something else is wrong.

If your motors stall occasionally, you try to run them too fast. Reduce their maximum speed.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Thanks Traumflug, it appears to have just been the max feed rate was way too high, I reduced it right down to 1mm/sec and now things are working just fine, motors are nice and quite, it also solved my other problem of inaccurate z end stop setting