Unlikely, but not impossible that the fault is in the tank. Would have to be bad solder connection or loose screw holding plates to ribs. Or perhaps the nut holding the BNC connector to the root rib. The last one critical to ensure proper grounding and reliable capacitance readings.
I think I would try with different converters (or swap left and right converters) before I would even dream of removing and opening the tanks.
Edit: I'm assuming Van's capacitance plates.
Never mind. I now see you already replaced the converters