I didn't touch my quad for a few months and when I got it back up in the air around Christmas I noticed the FPV feed had constant wavy lines going through it.
I honestly don't know if this was there before and I just didn't notice.

I do know that it is NOT tied to throttle.  i.e. it's not a filter issue, but just to be safe I bought an LC filter and tried it out, didn't fix the issue (went from PDB to CAM).

Looking for advice on what else to debug.  
The lines are not static, it's as if the video has alternating horizontal lines that are shifted sideways by 5 pixels left or right.  It's totally flyable but slightly annoying.

Can you post a video or screenshot of the issue?

No? I wouldn't know where to begin. Don't have a DVR or a way to record and upload video from RCA cables. Think old school VHS video where the tracking is off.

or take a phone and take a picture of inside Instead of your eye put phone camera

i've seen so far 2 types of such lines as you are describing either they are like
_
_
_
_

or like
////
////
////

one way or the other in my case this was related to crappy noise filtering from the battery (even without touching throttle).

thing to check... connect other source of signal (not from quad or other quad etc)
to see if this is inside of the quad or maybe you need to filterout the power in the gogles/screen.

Grisha, they are like
__
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__

Not sure what you mean by 'connect other source of signal'? Do you mean plug in something else to my goggles like a dvd player?

I think he means you can try and connect the camera video pin to your monitor's video input. just to see if the problem is related to your VTX/VRX or your camera/display. Then you can narrow down the issue...

I had this on an old fatshark vtx. When feeding the cam from the vtx 5V output the goggles showed pretty strong lines.

Changed it so the cam got 12V directly, and only signal wire connected to vtx, picture was clean.