In cases like this, I split the signal & feed the TV & my tester at the same time.
It helps if your tester shows RS uncorrected errors or dropped packets, I've found some instruments won't necessarily catch instantaneous glitches on just their internal pre/post viterbi error readings. (If you still have your Rover, that can do it. You can also use the Peak Hold function on the spectrum display to look for impulse noise that might be an issue.)
Which one? With 68dBµV at the wall plate, there may not be much signal coming from the antenna, particularly if there is a splitter in the mix. What's your noise margin with an MER of 27?
That may not mean much, as it depends on what parameters the TV is using to derive this indication. If it is a post corrected error rate, this will only show when the signal is corrupted sufficiently that the decoder in the TV is not able to make sense of the digital signal being fed to it.
The only way to determine for sure is to use a digital field strength meter to get traceable real world measurements.
Could be anything: I've come across electronic devices that are, or have gone faulty, electric fences, street lamps, power lines, faulty antennas, to name a few. If your antennas & masthead have not been looked at for a while, now might be the time. It could even be a change of propogation conditions that has selectively affected channel 9 only, however, this is usually outside the ambit of most homeowners.
You could try turning off every electrical item in your house, to see if this has an effect, then, if it does, turn items on one by one to see which one is causing it. In recent memory, I came across an electronic digital clock in an old oven that was causing havoc to some channels, so you never know what could be doing it.
For the last few months channel 9 (Freq 191500) with GO and GEM have been pixelating. Now for years we have never had this problem but I will fill you in with some Info.
1/ We live approx Half way between Mt Cootha Brisbane and the North Coast Sunshine Coast Tower
2/ We have 2 Antennas. One pointing Mt Cootha and the other pointing towards the North picking up all the Sunshine Coast Channels
4/ Using a Mast Head amp picking up all channels up the Sunshine Coast and all from Mt Cootha.
5/ ALL has been perfect for over 6 years. NO PROBLEMS WHAT SO EVER.
6/ Now we have issues with Mt Cootha Channel 9, GO, GEM, being freq 191500. What happens whilst its pixelating is the signal bar on my Terrestrial Sat box or all other TV's goes from 99% to ZERO and back again in a matter of a few seconds.This issue was mild up to two months ago but last night and up to weeks ago is becoming Vicious and pixelating every 5 seconds.
Question !!!! What could be wiping us out on Freq 191500 that makes the Signal go from Strong to Nothing in a few seconds and strong again. Please note all TV's in the house have this issue.