That flicker-fest definitely looks as though there is a poor ground connection somewhere. Also, logic gates can misbehave if you have inputs floating (not tied to 0V or Vcc) or outputs tied to other outputs or fixed logic levels, and one gate wrongly wired can affect all the other gates on the same chip.
The first thing I think I'd do on getting back to this would be to prove continuity from the negative lead of the power supply smoothing capacitor to all my grounds; then check that there were no floating inputs or tied outputs on the logic ICs. It's important to probe right onto the IC pins themselves, to include
every connection in the test: they are
all suspect.
Good luck finding it! And try not to kick yourself too hard when you do
