loud intermittent pops from DAC, possibly related to long USB cable from computer

when I bought my benchmark DAC 3 years ago, it made loud intermittent pops. people suggested it was due to the long USB run, so I got a 'USB active extender' 15 ft. cable which connects to a regular 15 ft. USB that goes to the DAC. the problem stopped - until now: I recently disassembled my system (changed the cabinets). when I turned it back on, the pops were back (fwiw: they only happen during the music, not when it's idle). I tried turning off the DAC, disconnecting and connecting the cables, but so far no luck. any ideas?