i have code P0300 and P0449 coming up

Question

I have a 03 chevy venture, when the problem 1st started, i would be driving down the road and the van would stall, as if i just turned off the key, i would pull over and it would start. I replaced the fuel vapor pressure sensor, PCV valve, spark plugs and wire,and the canister vent valve solenoid, check engine light is still on and it is still misfiring, it will go across town on flat ground, but when i try to go up a hill it will misfire till it stalls and it has to sit a bit and will start back up. it will sit and idle fine like there is nothing wrong, it acts up really bad when you put it in reverse. been broke down for 3 weeks now or more.