Is noise in a TL494 based push-pull converter causing low duty cycle?
Pretty much what I'd expect from breadboard - you are seeing 0V bounce and that will likely be the high currents inducing voltages in the large loop formed by the scope probe and it's respective earth wire. A really common problem faced when doing switch mode supplies. Here is a picture of what some folk do to avoid this problem: -



You can also buy scope attachments like this: -



Other things:


If you expect D1 to somehow magically do something, forget it - the internal diode in the MOSFET will be much faster (even if there was something useful to do (which there isn't)).
C1 (4n7 across drain the ground) is just a stealer of energy - you charge it up and then you discharge it hence an energy waster. If in fact you are trying to make a snubber to prevent spikes killing the FETs then I would recommend a zener diode in series with a conventional diode from each drain to the incoming power rail - at least then you salvage some energy in the flyback spike.


Anyway, they are your starting points as I see it.