how do i fix this?????????

I have a 1996 Hyundai Elantra 5-speed. It is missing badly at low rpm while in gear, the check engine light will flash, and I have to bring it to about 3000 rpm before its will stop missing. I have the codes (P1123, P0446, P0304, P0300, P0301). I have replaced the O2 senor, and fuel filter, plugs and wires. I have also cleaned the MAF, and sealed off any possible leaks. Oh, and there is a strong gas smell after filling up, and It will not go away till i drive about 100 miles. I have little money and would like to stop throwing parts at it