[old_tv_nut]

Hard to guess without more details of the circuit and investigation with an oscilloscope. One possibility is that the point where you are injecting needs a DC-restored signal in order for sync separation to work properly. So, you are seeing AC hum (perhaps a separate problem) plus you are seeing it un-synced until you have enough amplitude. Then when it is synced, maybe it's OK, but maybe not. You didn't stop adjusting at a good point or make the video long enough to tell what's going on. You need to adjust for an intermediate level where it syncs and see if it remains in sync or loses it depending on the picture content. If it's unstable with video content, it probably means DC restoration is needed.

Please publish more of the TV's circuit and where you are connecting the amp, if you can.