A very warm welcome to Beoworld and I am sorry to hear that you have put up with this problem for some years now.
When you say that the screen goes black, is that as if the TV is switched off or is there a very faint blank raster visable ?
As you are apparently still using the analogue signal and the sound remains when the fault occurs, it narrows the problem area down considerably. As the problem has apparently not been getting any worst, tends to lead me to think that there is a bad connection or dry solder joint after the 6Mhz sound filter.
I suggest as a first step you carefully view the screen when it goes black to determine if there is a raster or not.
Are you a technical person with electronics knowledge ?