Horizontal lines on display

So I'm really hoping to avoid a THIRD trip to the Apple store, as my 27" iMac is a pain to lug around. So, long story short, my graphic card failed, had it replaced at Apple, got the computer back, then the display failed, apparently a cord was damaged during the replacement process of the graphic card, so they replaced that cord, got the computer back.

Started up, everything worked fine, left it on for the day (as I usually would). Came back tonight and was planning on performing a restore to get my files back on the comp (I always wipe everything before taking it to the apple store), was having problems getting to the recovery mode, so tried to get into diagnostic mode, held down the D key on start up, I wasn't able to get into the diagnostic mode as the computer went straight to the regular start up, but when it got the desktop there were thin, uniform black lines on the whole screen. Reading around I saw some suggestions to reset the NVRAM and SMC, so reset the NVRAM and the black lines disappeared when the white screen came on, but then at some point while the progress bar was loading, the black lines returned. Repeated attempts to reset the NVRAM have been less successful, now the black lines are just there at the start all the time. I also reset the SMC, again the black lines disappeared on startup up and stayed gone all the way into the desktop, but then returned shortly after startup. Repeated attempts at resetting the SMC failed to repeat or improve the results, now the black lines appear at the white screen and remain throughout. I've also tried re-installing El Captian OS to try and resolve the issue of the iMac not starting up in diagnostic mode, but in spite of that neither D or option-D gets me into diagnostic mode.

Lots of backstory I know, but not sure what might be related to what so figured I'd throw it all out there at the start. Thanks for any help!