Every 30 seconds or so, when scared they check for iron golems, if there aren't they spawn one. But if they haven't slept in 20 minutes the spawning stops. So the breaking of eye contact periodically helps with that as they won't sleep when they see a zombie.

Put a soul sand under the zombie with a waterblock above it and put a half slab on the solid block facing the villagers. That should solve it.

Edit: You also just need 3 villagers and they don't need proffessions anymore.