Sorry if I'm asking the obvious...

Have you monitored the recordings through Sonar? If so, are their periods of silence while Sonar is recording that then get closed up post-recording?

Have you tried recording into e.g. Audacity or another wave editor? If so, does the same problem arise?

Edited to add - I use my UFX via a Thunderbolt to firewire adaptor on Mac and it performs excellently, but have never succeeded in getting it to work properly in WIndows using firewire, so for Windows I've always used the USB connections. Firewire in Windows is a very tempremental thing indeed.

Just for more information, when I record videos of me using sonar where I'm recording the web cam, DigiCheck, and sonar all at once, I get this exact behavior and have to mix together different outputs to match up to the real time video.

To replicate, start recording in DigiCheck, open sonar and begin recording. You will notice the issue on the DigiCheck files.

This happens using USB. I think the problem is outside of how the interface is attached to the computer.

@tlw
I have not monitored through SONAR.
 
@gswitz
All computers for recording tape or vinyl have no internet. Therefor no Netflix. This are computers just for recording. Nobody works on this computers. Work is done on our 4 large DAWs. (Recorded Data is moved over local Network)
 
I say:
A laptop that crashes by closing projects in SONAR has in any case a problem.
If you want good results use professional devices - and not a DELL with a bulitin firewire card.
 
The RME 800 is 9 years old.
At that time 2008 we recorded 1100 vinyl for the German National Library with Windows XP, Sonar 6, and Pentium 2 with 800MHz and this RME 800. NO HOLES.
And now it does not work with Windows 10, DELL, DUAL Processor and 2,5 GHz.
This is a shame. I would assume that the DELL should have a better firewire card as the very old computer for 9 years. This is no progress.
   
Now there is back the old working computer Dual Core 3,4GHz. The old computer has a very good firewire interface card and all works perfect.
 
I don't have the time to check why the DELL has such issues. And I don't can check hours of recorded material to find "holes". We are under pressure through our customers.
 

You have a solution! Good. Continue getting the work done.

You can send the Dell to me.....

I believe, that the firewirecard makes sometimes a pause (cable, hardware, overload, driver ...?) and in this pause SONAR stops intern to record the stream - then continues recording as soon the firewire delivers new data. Actually there should be a silent passage in the recording.
There is no other way to explain why input stream 3:17 is shortened to a length of 3:15 with a hole or "cut" in the stream.
My problem is however to exclude this in future. Therefore a "cancellation" in the recording or a warning would be helpful, when data is not received for a short moment.