Have you tried re-soldering the jacks? Sometimes they develop halos of cracked solder around the connections, and all it takes is a reflow of solder on the connections. (at the same time add just a bit of fresh solder to supply a little flux to the re-solder connection.
What I've done on a number of amps with pc-board mounted input jacks is de-solder the old jack and put a SwitchCraft jack on the chassis and run wires from it to the pc-board. Fixes the problem and isolates the jacks (and associated wear and tear) from the board. Sometimes there isn't enough room between the chassis mounting location and the board to allow it, but where you can make it work it significantly improves the reliability of the amp.