Hello everyone,
since i hadnt gotten a reply soon enough, ive taken this iMac in to my supplier and tested it with a bunch of known-good-parts.
I've swapped the LCD (twice), V-sync Cable, Inverter Board, LVDS cable, even all of these items.
The vertical wavy noisy lines remained visible.
After logically assessing that this cannot be the video-card (digital video artifacts or any other typical GPU related issue generally looks totally different, plus this has a sort of 'analogue-ish' look to it: the lines are like an old CRT-screen that has gone bad, or an old TV that's got bad reception on the coax cable) Ive concluded that it must be the Logicboard, and probably the LVDS connector on the board.
I am pretty darn sure i havent ripped the cable out of the connector when opening up the iMac (i've opened up imacs at least a million times before and know what to watch out for) however, maybe this specific connector had already been played with before and gotten a little sensitive..
just thought i'd let you guys know; i know how darn frustrating it can be to google something that is practically un-google-able (you have to be sort of general in the searchbox, yet specific enough not to get all kinds of bogus results like ads, manuals for decades-old macs, or bulls**t websites) and come up with zero results after spending hours on it.
People need to follow up their questions on here more, and post more in general.
im all for sharing the knowledge, let's keep it up!
oh by the way, in the end, my customer didnt mind the defect and said that she would never even have noticed if i hadnt told her about it.. funny