I guess you’ve already double checked all the actual wire/solder connections? Also, make sure the 1-14 pinning on that display corresponds with the standards (like in Thorsten’s docs). If it has a dual row 16 pin header or whatever, make sure it’s laid out in the correct order for the core’s pins. Mine never were, so I had to wire custom ribbons. I’m guessing pins 1-14 for 44780’s normally correspond with the same chip destinations, regardless of the pin layout.(?)
I think the row of black lines is usually from the display not getting all the data signals, if it’s already been loaded with MIOS. The intermittent thing you describe sounds like an actual glitch in the core wiring or parts rather than the display, but I could be wrong. I had that “rapidly plug it in to get different results” thing a while back.
Also, when I’ve failed to make all the required pull ups/downs and left stuff floating, the display flickers so bad that you can’t tell that it’s just jumping from parameter to parameter or changing values. It almost looks like an LCD problem.
                         Good luck with it!