You can try the AGC also. What happens is the set gets
say a weak video amp & the pix is washed out. Turn up
the AGC & it looks OK but is on the edge of IF overload.
Try unhooking one wire from your source so you get a snowy
pix & see if it still does it.
the audio detector/quad coil can also get rid of it sometimes
but that is a LAST resort. They are old & dry & break up easily.
73 Zeno