Have you tried monitoring your guitar with headphones directly on the Tascam 1082? Is the distortion there? If so, it would seem to point to the Tascam or the guitar. If not, it's something with your audio driver/config on the computer, or like suggested above, the trim on the Sonar channel you're using is way too high. Have you tried with a new, clean (not from a template) project in Sonar?
Have you tried a different channel on the Tascam? A different source, e.g. mic, CD audio, etc.? Are you using channel 8 with the guitar switch? If so, have you moved the switch recently, i.e. move the parts and possibly restablish a good connection between the contacts. Have you unplugged and replugged all the cables for the same reason?
If you're in channels 1 or 2, maybe the insert jacks have a bad connection. Try inserting/removing a 1/4" cable a few times.
I would assume that when you set a sample rate in Sonar, it changes it on the Tascam appropriately, but if not, do the sample rates agree? Do you have a clock option? Internal vs. external? My Echo Mia distorts if my dbx 386 isn't the clock source because I can't change the external gear in my case from the computer. Of course, here, my connection is S/PDIF, not Firewire or USB.
