I found a difference in DSU/CSU configuration between the working circuit and the one that was flaking out, and changed the circuit that was in trouble to match the working one. The difference was that the CSU/DSU of the good circuit was set to DACS mode, the CSU/DSU of the flakey circuit was not. I have confirmed the settings with Sprint and they tell me it should be in DACS mode. It seems to be working fine now. The odd thing is that I haven't changed this in literally years, and up until a month ago it didn't seem to cause any problem. About a month ago we had a spate of this thing going into loopback by itself and then it went back to working until this morning. I guess it's possible something may have bumped the switch. At any rate, it's working again.