Most intermittent operation faults on all layouts, not only DCC ones, are caused by bad electrical connections and poor wheel pickups on locomotives. Poor electrical contact causes problems with sound decoder operation in particular. Sound decoder installations usually include a capacitor to store energy to keep the sound going when the decoder briefly loses electrical contact.
When the track is dirty or corroded, it can't transmit the electricity and DCC signal needed to run locomotives. When the locomotive pick ups are dirty or corroded, they can't pick up the signal from the track. Check your track cleanliness and quality of wheel pickups to make sure you are getting the best electrical contact possible.
Digitrax' DCC communication strategy performs exceptionally well in a less than perfect environment. Digitrax command stations refresh (re-send) commands to decoders often enough to mask bad connections until they are really impossible to work with!
Clean wheels and clean track will make your operation more reliable and more fun.