Increasing vertical deflection makes image shake

[castlesteve]

I've got an 8liners board question:

Scenario 1:
Tube impedance - 1.7ohm H, 10ohm V.  image is stable at full screen (640x480i, 320x288) etc

Scenario 2:
Same board, Tube impedance 1.7ohmH, 6.5ohm V.  Image is stable at 320x288, but is shakey at 640x480i.  If I reduce the vertical size to about 2/3 of the monitor, it is dead on clear.

So obviously, my vertical yoke impedance is affecting the full screen ability.   By adding a 1-2 ohm 10W resistor to the horizontal yoke circuit (in series as suggested by 8liners), will that in effect correct my shaking issue?  I realize the horizontal circuit does supply power to the whole coil.

Thanks,
S