I own a 28mm 2.8 AI lens and it's excellent. Also have a 35mm f2 AI and it's also of a high standard even wide open. I wonder at the reason you purchased a 28mm f2 lens?....These kind of fast wide angle lenses were designed primarily for available light hand held photography. If most of your photography is daylight camera on tripod stuff, you'd be better off with the 3.5 version.
Finally some of us believe those early Nikon lenses made for the "F" etc., were made to a higher standard than the later AIs range. I believe production was shifted outside of Japan by then. Possibly "building down to a price" had entered the equation by then, replacing the previous maxim of excellence at all costs to combat the German Leicas?