Try a drop of contact cleaner first, but I doubt that this would provide a long-term fix. The more likely scenario is the leaf-arm contact has finally worn out and no longer has sufficient spring action left to anchor the plug tip in place. This is caused by fatigue of the metal leaf-arm which essentially acts as a spring-loaded connection to the tip of the plug.

They're fairly easy to replace once the keyboard is opened. And, the older ones were held in place with a hex-nut, while the electrical connections are just a couple of wires soldered to tiny lugs. Unfortunately, some of the plugs are buried deep beneath printed circuit boards, which have to be removed first in order to replace the jacks.

Good Luck,

Gary