SgtIk3

So over a year ago I started noticing my car miss under load at low RPMs. Once it gets up to 2500 and above it was run fine. If I baby it when taking off it does fine as well, but as soon as you try to push the gas anything over 1/4 throttle it will start missing until about 2500 then it will smooth out. I replaced the MAF Sensor, plugs, wires, air filter, cleaned throttle body, all electrical plugs and connectors but nothing has help. As a side note, that may help as well...I did notice that when I put 89-91 octane instead of 93 it would make the miss ALOT worse. I didn't do this purposely but on a few different occasions I was almost out of gas and had no choice. About a month ago I bought a big bottle of Gumont "Severe" fuel system cleaner just for ****s and giggles and noticed that I got literally 100 more miles on that tank of gas and the miss was about 80% better. Once I filled up again and didn't use the fuel system cleaner the miss was back. Ive been testing and replacing electrical components from the start but Im almost convinced its fuel related. Initially I had a CEL which was Cylinder 1 misfire. Only 1 time over the year have I gotten a "Multiple Cylinder Misfire". Ive been trying to get the CEL to come back on by driving up steel hills in high gear and lugging it, since that is when it misses. I got it to flash a few times but never stayed on. Could this be a bad or clogged fuel injector? That's my next guess. Any suggestions or opinions would be greatly appreciated. 2004 IS 300, 5 speed, all stock.