[old_tv_nut]

Poor interlace results when horizontal sweep waveforms bleed into the vertical sync section. Because of the high currents and voltages in the H sweep, this means that grounds are critical. Many sets have questionable interlace under normal use without added circuits, but adding a baseband video input may have created a ground loop for horizontal sweep currents/voltages.

It may help to do some trial-and-error experimentation of where the added video input circuit is grounded, but there is no guarantee.