Some ideas:
-Make sure no ground screws are missing, that go through tabs on the PCB. If they are all there, make sure they are tight.
-This is a pain, but remove the PCB and check the solder connections, especially on the jacks/pots. You'll have to remove the knobs and washers on all the pots first.
-Sometimes due to age etc corrosion forms around part legs, and sometimes you can even pull them straight out if it's real bad. Resolder bad joints if you find them.
-Make sure no ground screws are missing, that go through tabs on the PCB. If they are all there, make sure they are tight.
-This is a pain, but remove the PCB and check the solder connections, especially on the jacks/pots. You'll have to remove the knobs and washers on all the pots first.
-Sometimes due to age etc corrosion forms around part legs, and sometimes you can even pull them straight out if it's real bad. Resolder bad joints if you find them.