timing belt changed now will only run at high RPMs P0300 code

[Josh39]

I have a 2005 and a pulley to the timing belt broke in january. Im in process of fixing the car now. We changed the timing belt, pulleys, and water pump. after the change i now have P0300 (random misfire/multiple cylinders). Now the car will start but heres the catch. you have to give it some gas to get it to fire. if you let off the gas it will die instantly, but if you hold the gas and keep it above 3000 RPM she sounds like a champ. I the changed the plugs and wires and Valve cover gasket. Problem still remains. I notice the air hose going to the air filter box is dry rotted. Could that be the issue? Vacuum Leak? I cant really check for a vacuum leak since i cant get her to idle. I was told that since she does fire that it isnt a valve or head issue. what can i do now to try to get her back on the road? Thanks in advance.