Elantra XD (2001-06) :: 2002 - Shudders And Doesn't Accelerate Well When Going Uphill

I've been having problems with my '02 Elantra. It shudders and doesn't accelerate well (or at all) going up hill, or attempting to accelerate quickly (like passing a car on the interstate). If I try to push it through these situations it will run worse, engine light flashes, and sometimes will stall. In the past, when it stalled, it would start up again in about 30mins or so.

Codes it has thrown were P0304, P0303, and P0420.

When I took it to mechanic, he really couldn't figure it out. He ended up testing and replacing fuel pump motor in November. Got it back and within a few days it was throwing same codes and having same symptoms.

Everything else from this point as been DIY. I bought a manual, researched these forums, learned as I went, and did the following myself:

I changed the spark plugs and wires. Still ran as described above but at this point when the engine light comes on, I am only down to the P0304 code right now.

Thought perhaps it was a vacuum leak. I have very few tools, and Autozone said I could use seafoam as a means to tell if I had a leak if smoke came out of vacuum lines as I put it in. I tried this, none came out of vacuum lines that I could tell. I drove it after seafoam, some white smoke came out tail pipe from solution but dissipated after driving a little while, and then I had the shuddering issues and loss of power while driving the interstate. It literally would act like I wasn't pressing the gas peddle, then suddenly surge as if it realized I was... engine continued to run until I pulled over and then it stalled. Normally it would start right back up after 30mins or so. But this time it didn't, I had to tow it home.

With a crap load of starter fluid and jamming the gas pedal, I finally got it to start. It ran horribly at idle, had a weird knock. So I didn't drive it and did the following instead.

I replaced the coil pack.
Replaced crank position sensor.
I confirmed that I have spark on all 4 cylinders.

There are no obvious oil leaks (nothing on garage floor and I don't see any obvious signs of wet oil on engine or smoke when it runs)

Temperature is fine, coolant level is normal & hasn't needed refill or topping off.

I pulled the fuel pump again, noticed that the fuel sock had holes and was full of muddy/pasty type crap. Gas was dark, cloudy in spots, and appeared to separate in some areas.

Replaced fuel sock.
Syphoned gas.
Thoroughly cleaned out gas tank.
Put premium gas in.
Manually cleaned fuel injectors.

Reassembled and it started right up, runs strong, no more weird knock, but the other symptoms start to reappear... the shuddering, loss of power, and stalling. Its intermittent... sometimes when I jam on the gas peddle, sometimes when I'm cruising at low speeds. I can force it to stall by jamming on the gas peddle to the floor, acts like it's choking, engine light flashes, lose power, then it stalls. Pretty much every time I push it like that.

I can continue to limp it to Autozone for codes, but the engine light currently isn't on.... and it usually takes it a hard drive pushing it through the symptoms for the light to appear, and it's always been the P0304 code at this point.

Is there anything else I can do from home with no special testing equipment?

Is there a part or sensor that would cause these symptoms and NOT throw a code other than the misfire?