Amp - Quiet left speaker issue

The left speaker is more quiet and the sound crashes. It can improve (but not always) after 5 -10 min. More often I have to slap the top of the amp (Marantz PM7200 I bought 18yrs ago) to get the sound back to normal. The issue persists after bringing this amp twice to a local repair music instrument shop.
1/ Any idea what the issue is with the amp? (I tested speakers, cables and the issue comes from the amp)
2/ Could turning-off the amp directly damage amp parts/components? (instead of putting on stand-by with the remote control before turning off the amp)

Thank you for your views.

Since we are dealing with electronic components.They most likely will break at some point.

It might be the Power amp board…? Not sure of course unless making a component level testing.
Found a Marantz PM7200 (Power Amp Board) & (Protection circuit board) Upgrade Kit being available so it could indicate that there might be something.