When I was using Ardour about 4 years ago I had no such issues.
Were you using the Behringer interface at that time? It seems there are a lot of reports in web forums of xruns with that device, even on windows.
Jack D-bus interface. So I tried and ...!!! that made a difference.
That probably indicates that you used different jackd settings when you launched jackd compared to the settings when using jackdbus.
I do not see where you ever provided the settings you use. Have you tried running Ardour using the ALSA backend, so that Ardour controls all the settings (without jackd at all)?
there is no change in priorities (or nice levels) in my process table
What tool were you using to check priorities? The applications you are using have a combination of realtime and non-realtime threads, so you would probably have to use something like htop to display all the threads. If you use top I think that will only show the priority of the first thread started. I'm not positive about that, but I think that is the case. What argument did you use for -P to set realtime priority?
persistent rumor on the net that wifi causes xruns
Some wi-fi drivers monopolize the CPU core while attempting to scan for networks, or at least did in the past.
Are you running the default SuSE kernel, or the kernel-desktop variant kernel which is configured for slightly lower latency?
For problems like this which do not occur right away but only occur after some period of time it can be useful to notice whether the xruns occur at the same interval each time, e.g. do they occur exactly two hours apart, or every hour, etc.? The OS will be running periodic background tasks which should be set to run at low priority to not disrupt your work, but something like scanning for software updates, or indexing the files on your hard drive, could cause problems. Probably not very likely, something to keep in mind as you keep investigating.
You should also check the system messages file for any indication that the USB driver reported an error communicating with the device. If sometimes the audio device won’t respond when requested