Ferdinand,
Latency can be a real pain. I usually get an office computer, mainly Dells, from the local university surplus. Fortunately they are cheap since they are old they tend to die after a while in the shop.
Max Jitter was up to 174000
That's enormous! The worst I've ever seen is half that, but It sounds like the new parts might have that in check.

On this latest computer a software update two days ago halved it.
I also tried to do Gcode syntax highlighting on Notepad++ it was frustrating and I moved on. I still use it on Windows as it seemed to be the best of the bunch. My new LinxCNC install is on Debian so I'm getting used to Mousepad, I'll see if I can install Kate. I'd really like to be able to do some light programming at the machine like figuring out HAL files and some Python M files. Can you post the file here? I'm sure others will find it useful and help with the bug reporting.
What's your spindle drive setup? I have not attempted to do LinuxCNC controlled variable speed yet, especially since I've had two Minarik DC drives fail on my mill spindle. I made a simple power supply and it's running full speed now, which is OK since I'm mainly doing aluminum with 8mm or less cutters. But I sure have spent a bunch of time reading the forums here and at CNCzone and maybe some new eyes can help you figure it out.
Regards,
Otto