Looks jreat I- didn't think you'd get it all in.You shouldn't need the 555. Are you using current limiting resistors with you LED dsiplay ? - otherwise you'll take far too much current ...
A medium capasitor (?100u) across the zener may help...
does your mod ( display via 330 to the arduino +5) mean the 7805 output isn't connected?
When you say you measure 4.96 from the psu, which one do you mean? there are 3!
Having done the mods, is the display bright? The 330 will seriously limit the display current.
I suspect much of the noise comes from the high-frequency internal multiplexing of the display.
Try adding >100uF across the output of my 5v psu, driving the display + from that, and disconnecting the 7805..