Define 'playing up'.
Those stats are about as good as it gets. Below 6dB SNR is where the problems usually begin, and at 9dB you're double that (log scale and all that...)
This said, 9dB isn't *that* high - if it's dropping, then something's causing it to get suddenly worse. Try the master socket (unplugging extensions) to check internal wiring... and check you have decent filters.