Hi again.
Just to test to see if it improves anything, I used some zinc sheet metal to shield the control box.
Earthed and everything, it did not make much difference.
So I started to disconnect components, and removed the vfd from his own shielded box, and then unplugged the limit switches fom the breakout board, and all was left was the pc with the drivers, sources and steppers.
The steppers are earthed at the driver end, everything at a star point as recommended. Checked all shield with the multimeter, and it.s all good.

The only thing done questionable is that I have one stepper cable, which I measured wrong, and had to join it to another bit, each of the 4 wires twisted and taped, and did the same with the shield, put some aluminium foil on top, and taped all over it. Not great, but it should not be a bigger deal than any other type of connection.
Also the limit switches are the cheap lever type. Even if not the best, they are not connected to anything now, and still have issues.

I removed all flexible couplings, and have the motors running without any load.

THE PROBLEM: randomly, the motors stall, all 3 of them, some a more often than others. Simply keeping pressed the keyboard arrow keys, they run ok, for 5, 10 even 30 seconds, then at least one stalls.
I,ve loaded a default g code, those coming with the software, and the same problem. I changed the motor turning config to acc 50 and speed 300, 400, 500, still have the problem.

I.ll remove all steppers tomorrow and connect them directly to the drivers. At least to exclude the cables as well.

The motors are 5.6A and the drivers set to 4.9A Peak, 3.5A RMS.
The drivers are DQ860MA and wired like this: +5v to PUL +(+5v), and bridged to DIR+ and ENBL+, then PULL- and DIR- go to the pins 2 and 3 for example on the BoB, ENBL- is not wired. The other 6 terminals are 2 for the source and 4 for the stepper.
The breakout board is db 25-1205.

I.m working on the metal cases, control box and vfd box, suggested by Rob and Clive, but I can not go further with the design and construction, until I figure out the cause of this stall.
I.ve already tried to separate the components and to shield all boxes and ground them. I rechecked all wires, including the drivers, and rewired those with twisted cables inside the control box. This wires between the sources and drivers and the BoB are not shielded. Everything else is.
I had a touch plate fitted and disconnected it as well, as it concerned me that the wire connected to the spindle, and to the chasis as is all conductive materials, sends a ground signal into the input pin of the BoB and might cause problems. Does it?


I.ll even remove tomorrow all other wires from the drag chain and leave only the steppers. It might exclude something else.

BUT I higly suspect the BoB.
What do you think guys? What am I missing?