One quick thing to try - a stopper resistor in series with the grid, positioned close to the grid pin, to act as a VHF stopper. The value isn't important, something in the 50-100 ohm range might work. Also, if the plate supply isn't decoupled, add a series resistor and a capacitor to ground. Try 1k/0.01uF to see if it makes a difference. 73, ian K3IMW