low volumeincoming audio signal

I have a PreSonus FireBox Digital Recording Interface 24BIT 96K Firewire Recording System. I use it on a windows XP desktop computer.
The computer is in great shape with lots of power. It is only used for recording and light graphic work.
Recently I have been having trouble with the audio signal. Some days the incoming audio records normally at the correct volume.
Other times the volume of the incoming is very low. I have done testing with other mics and instruments and cables. No changes in setup
of the computer of the audio interface. I have done all the usual things like turn off and on the computer and audio gear.
None of these tests have helped find the signal volume problem I’m experiencing. Any help or information with this would be appreciated.
Thank you, DC