19 year old TV sound gets lower & changes to white noise. Picture shows diagonal background lines and gets fuzzy when this happens. Sound and picture return to normal after TV is off for a while.

Obviously an analogue TV set!

Sounds like the station is drifting out of tune as the set heats up. Could be many causes – leaky capacitor, dry joint, etc.

Unless you're an electronics technician or engineer with a schematic, there's little chance of fixing it yourself, I'm afraid.

Time to go digital?

Daniel