Is this screen broken?

[mechanicus]

Hey folks,

thanks a bunch for this awesome forum! I've learned a lot already reading around here.

I'm going to put a Pi Zero into a Gameboy Color. I got this 3.5" screen off ebay because I read a recommendation stating it would run off 5V:
http://www.ebay.de/itm/282124728983?_tr ... EBIDX%3AIT

What I got was this board:

20161204_191032_1480875058628.jpg (3.89 MiB) Viewed 2242 times

I had it up for testing a few times already and it seemed to work well with an external 12V PSU and the RasPi's Composite video output.
Today I wanted to do a final test before modding it to 5V power supply, but I got no image.
Only when I disconnect and reconnect the power plug, I get a blurry image like this for about a second, before the screen goes black again -

20161204_190945_1480875058365.jpg (3.21 MiB) Viewed 2242 times

I can reproduce this, so it probably isn't a slack joint or something. The multimeter shows a stable 12.2V at the power input.

Any ideas on what the problem might be and how I could possibly fix it?

[mechanicus]

Funny thing:
Screen doesn't run off of 12V anymore. But miraculously, 5V out of the Raspi seem to work now, which wasn't the case earlier.
Now I only have some slight stripes going over the screen. Might be some shielding or config issue.