There have been several accounts of similar issues documented on, links are below for reference.

http://www.rme-audio.de/forum/viewtopic.php?id=12083
http://www.rme-audio.de/forum/viewtopic.php?id=12664
http://www.rme-audio.de/forum/viewtopic.php?id=15814

In my case, the gain control on mic inputs 9 and 10 is intermittently unresponsive.  On occasion, turning the gain upwards from 0db has no effect whatsoever until anywhere between 30 seconds and several minutes have elapsed, whereupon a loud pop occurs and the input resumes functioning normally.  Mic inputs 11 and 12 do not exhibit the same abnormalities. 

Turning the unit off and on again seems to correct the problem.  I have tested the unit with all available drivers (currently 1.0.25.0) on two separate systems and the issue is present with each and every combination.   I would be very grateful of your assistance in resolving this matter as I'm concerned that the unit may have developed a fault and be in need of service or replacement and is a little over a year old, please advise on how best to proceed.  I'm a UK customer and could contact my place of purchase but would need confirmation from you in the event this is a fault.  Thanks in advance.

I'd also be interested to know if anybody else is experienceing exactly the same problem and if there is a simple fix involving drivers and/or firmware.