/6 sputters under load

Hey guys,
Here's my set up- 75 Dart Sport /6 Holley 1 barrel. Plugs, Dizzy cap, rotor and wires fairly new, timing set at 10*

Problem- Engine runs good and smooth until I have to give it a lot of gas under load (like up hill or passing) Under load engine starts to sputter and loose power. Seems to do it more/or worse at higher speeds.

Thanks for any and all help, Matt