like JP said,maybe swap cables,(2) does it happen if your computer is only in front of or to the side of the amp where an internal mic would feed(3)is it at ALL volume levels,maybe try turning down the output of both the computer/amp;see if it's a balance problem.We're gonna start a rumor you're too hot to handle Koda lol
any chance your signal is being fed to another amp/monitor/pa-and THAT is the problem?