If the 'good' channel is good with one input but bad with another input then the input part of the circuit is bad, not the TVP.
Did you lift the input coupling capacitors when you did this experiment or did you simply connect wires?
Please re-send the original oscilloscope captures but with correct triggering so we can see a clear video signal.
Please also measure the power supply noise. If you suspect the power, then check it :)
BR,
Steve