One axis loosing it completely

Hi everybody,
I will try to explain the situation the best I can.
I build myself a CNC Mill and it ran great for many projects, no issues at all. One day, I started a job and I found the Y axis completely crashed in the ballscrew bearing block with the drive in error. 

I get no problem while jogging only, when executing a program and it only happens after a couple of hours of the job. I tried lowering the max speeds, accelaration, highering the step time (like i said the machine worked great before so I don't see why it could have helped but I tried anyway) but nothing works. The weirdest thing is that it seems that everything works great and out of nowhere the y axis decides to go ram itself directly in the hard limits (it seems, am not quite sure if it's gradual or just like what I just described). Also, something worth mentioning the problem cannot be mechanical slippage I triple checked and everything is tight.

I tried many times to execute the program above the stock without any load on the steppers but I get the same result. My theory is that I have a problem with either my pcie to lpt or my breakout board but the weird thing is that it's only a few hours in the job that the problem occurs so I don't know why these components would work at the beggining of the job but not in the middle of the job.

I am really lost here I hope someone experienced the same issue and can help me. If you have any questions let me know. 

I am running linuxcnc 2.9.3 on a relatively recent pc that i added a diyeeni PCIe parallel port. I am using a nema34 closed loop stepper motor with an HBS860H drive on the Y axis. The breakout board and cable came from amazon nothing crazy there. If you need other more specific information let me know and thanks for the reply.

Interesting, I will check carefully all the encoder connections to see if anything looks suspicious. If I find nothing I will try to swap two motors to see if the error follows the motor.
Thanks for the idea!

In my experience pushing open loop steppers to extremes, they only loose steps when they get hot which can take some time when operating. At this point your drives may be unable to compensate for position based on the encoder feedback due to lack of torque.
There are many variable to consider if this happens,

Found the issue!

Checked all encoder connections, everythings fine. Removed the encoder cover and found small aluminum chips, removed them carfully and sealed it of better. Still the same issue.

The problem turned out to be the powersupply I suspect the thing was overheating after a while and shut off so the motor skipped some steps.

I will add cooling fans with fin filters to keep aluminum chips out. Thanks for the help I am happy to have cleared the chips from my encoder.