You probably solved this by now, but check the O2 Sensors and the Manifold Catalytic converter. I had a similar problem following an Automatic Transmission swap - the car would start, run, and drive but any sort of load would cause it to stumble badly. This cause the random undiagnoseable (really uncorrectable) misfire codes. Drove me nuts.
I got an inspection camera to check if the intake valves were loaded up with gunk for some reason ( it had 325000+ miles at the time) but they were clean. On a lark I pulled the upstream 02 sensor to look at the exhaust valves, and found nothing there either. So I looked in the other direction at the catalytic converter, and saw a lone white spot about 1.5 -2 inches (35-50mm) in diameter, and decided to pull it thinking it was "starting to go". Turns out the white spot was the ONLY part of the cat flowing. The ends looked all nice but the core of the thing was melted, and therefore clogged.
(I now have a Toyota specific "test pipe")
Replacing the catalytic converter after verifying it was clogged solved my problem. (Note that had I NOT previously replaced the O2 sensor, I would have done so with the new cat because a bad O2 sensor is the fastest way I know to kill a catalytic converter.)
BTW I still drive the Camry and it has 355,000 Miles on it.
Sunday, March 30th, 2014 AT 9:14 AM