After many years it's normal for a layer of dust to gather on circuitry, more so on high-voltage components such as a flyback. Heat buildup is the result, increasing gradually.
Besides which there's the attraction for carbon particles (from sources in the tv or the air), creating a mildly conductive avenue for electrons. Not necessarily to the point of arcing, but able to draw down voltage thus hampering proper operation.
Cleaning is difficult around congested circuitry. A tiny brush vacuum is on the market. High pressure canned air is another thing to try.
You may get lasting success if you install a fan. Space is probably scarce so you may need to mount it outside. Drill a hole in the cabinet perhaps.