What is wrong with my 92 accord? my fuel pump died on my over a year ago, so i had it replaced, along with?

fuel filter and fuel pressure regulator. spark plugs and wires were all changed not long afterwards. my battery was about to p the warranty period, so i went and traded it out at wal mart for a brand new one. ever since then, the battery light has been on. occasionally my radio volume will just skyrocket on its own. no biggie, but still. anyways, the main problem with this car is that after the fuel pump was replaced, the car started to run like crap once it got down to a half tank and would start stalling when i slow down. I took it to a couple shops and they didnt know what was wrong with it. Im in school so i couldnt leave it with them for a couple weeks to let them dig into it. I've just been keeping her full and putting it off. my engine light comes on every now and then and i've been ignoring, but i just checked it and it has a code 12. whenever it does get down to close to a half tank is when it starts acting weird. it idles fine, just stalls when i slow down. it starts right back up but if i have to slow down a few times before the next gas station and it stalls on my a few times, it takes a few tries to start back up. I know its pretty difficult to diagnose over the internet but hopefully these are common issues with this car. I'm not too good with cars but with some good directions i can figure it out if I have the proper tools.