QLN (quiet line noise) is a measure of the background noise on the line. It can only be measured at the time of establishing the connection, before sync is obtained. After that, the values reported by the modem don't change until a resync or reboot occurs. So it wouldn't be any help in the circumstances you described, which were a temporary large increase in interference which didn't cause a resync.
If you want to get a modem/router which provides more comprehensive monitoring data than the TG582N, then the D-Link DSL-2740B is a good one at a reasonable price. Or you could spend a LOT more and get a Billion 7800DXL. Those are just my suggestions, I'm sure there are others.