Strange recording issues on X2

G'day All,
 
I'm posting here in hope that perhaps someone has had this issue before and knows the fix, or with some combined heads we might be able to solve it.
I'm using a PreSonus FireStudio on Windows 10 x64 into Sonar X2 (fully up to date).
The strange thing is that when I record, after a period of time (usually under a minute or so) the recording develops a distinctly digital distorted sound to the recording which will continue until it stops.
I have tried changing bit-rates, sample site, latency settings and even switched between ASIO and WDM drivers, all suffering the same problem. 
Occasionally starting and stopping the recording makes the problem stop, but then it re-occurs very quickly.
The other thing to note is that when I monitor the input directly from the FireStudio, it is not distorted or have any problem, it's only when routed through Sonar that it develops this issue.
 
Has anyone else come across this? I'm really stumped with this one. Thanks in advance.

Thanks for the reply,
 
Its a FireStudio Tube. It's quite likely it's a driver issue as I am aware that they are discontinued and not officially supported. The website does indicate that while it's not fully supported, it is reported to work. I just find it strange that it sounds fine when monitored through the FireStudio.
I have also tried the ASIO4ALL drivers to the same effect.
 
I was just putting it out there to see if anyone else had come across the same issue.