Well, another test, fiddling with width of step pulses (in Motor Tuning), same results.
@Graham: No broken wires. Let me "explain" how the system looks electrically/electronically:
Dedicated enclosure with 4 drivers, power supply and BOB (5 Axis Mach3 Breakout) which works as it should, on input side powered by power supply that supplies motor drivers/motors, on thr output side powered by USB cable connected to computer MB, parallel cable M/F for step/dir/switches/... between BOB and PC.
In case of using Arduino, only Parallel cable is shifted from PC to Arduino (homemade PCB holding Arduino Nano and DB-25)
With GRBL on Arduino, everything is SPOT on.