Did you check the line-out connection on the right channel and also try a different cable from the DAC to preamp/amp? If that doesn't fix it, sounds like it may be a loose internal connection. Might be worth opening the case and checking to see if the internal cables are all fully seated. Also, poor solder joints can also work intermittently, though these are rarer these days than in the past when circuits were point-to-point soldered.
If checking the cables and checking the internal plugs doesn't do it, I'd suggest you just ask for a replacement. That assumes you like the sound when its working. NO manufacturer has 100% perfection from their production line and you also have the possibility that something could have been rattled in transit.
