Had to remove then take apart the control grid rheostat. Cleaned it with deoxit.
I also found a problem where the RF input did not match the schematic. Instead, the input of the first transformer was fed through a 75 ohm pot ganged with the control grid rheostat, thereby working as an RF attenuator. The wiring for this was factory, not a goof-up later in the radio's life.
I removed this and tied the first RF transformer primary to the external antenna jack and the radio is now working at its best possible sensitivity.
It is very sensitive in fact, and i am now happy with its performance.