Hello, all.

I'm new to this forum, my car is a 2001 GS300 with the 2JZ and auto tranny. Currently the car misfires under load, yet it purrs at idle. I get misfire codes PO303, PO305 and PO306, nothing leading me to conclusively say its ignition, fuel or timing.... The car has 228k miles on the clock.

As of right now, Ive replaced or fixed the following:

The spark plugs were replaced with NGK Iridiums (gap set to 44 thousandths)
The MAF was cleaned
Fixed any obvious vacuum leaks (replaced intake gaskets, fixed the intake muffler with some epoxy)
Replaced the timing belt, cam seals, water pump and fixed the vvti O-ring (I understand that I shouldn't have done this, but the O-ring was just absolutely puking. I put it together correctly.)
Replaced crank sensor
The coils are new (replaced before me, they are non-Toyota/Lexus brand)
The O2 sensors have been replaced in the past (non-Toyota/Lexus brand)
The dude before me also replaced the harness and connectors for the coils... But he just used stereo quick splice connections, so I got my soldering iron out and fixed those connections too.
I followed the harness back as well and fixed any broken wires or DAPO wiring fixes.
I replaced the alternator, and the battery is newer
I checked the ground connection to all the coils and spark plugs, and even added a chassis-head connection to improve the resistance reading.

Also, after replacing the spark plugs, the problem went away for a day then came back the following day.
I am now at a point where I do not know where to go next... Any ideas? Thanks in advance.

 

You've checked ignition, so now check your timing and check your fuel delivery to those three cylinders.

 

Did u fix the issue and if so what was it? Very similar to mine

 

I had the same issue , you need to clean the spark plug connection area and the coil boots , its the oil from valve covers that goes there and effects the resistance .

 

You did the right thing by doing timing and redoing all the seals there, since that's a pretty big suspect of misfiring under loads.
Did you actually vacuum test it, or did you just replace the biggest culprits? What're your fuel trims like? Whether your fuel trims are normal or not will definitely point us in the direction we need to go here.
If your fuel trims aren't normal, then we know what the problem is, easy fix. If they are, then we know what the problem ISN'T, so that's at least something.

All in all, though, if you're going on a wild goose chase replacing things and it's still doing the exact same thing as before you replaced all this stuff, have you verified that the problem isn't the ECU? It's good to rule that out early, than to get readings that make you think it's a bunch of other things and end up throwing the parts canon at it before finding out it's the ECU after you've basically refurbished the whole engine.