I would inspect all parts that comprise the ohms path - I would first look for signs of grime and residue and thoroughly clean the board. I would then check the input jacks for any signs of bad contact or residue.
After that, I would go to look for the parts. Are you sure the varistors RV2 and RV3 are in good shape? You can lift one of their legs and see if the issue disappears. Also, does ohms read higher resistances accurately? If not, I wonder if C3 is leaking or even Z1 might be faulty.