3.5 will not rev over 3500 rpm. PO 449 is the only code stored. It will not rev over 3500 cold or hot. After driving 10-15 minutes seems like the transmission is searching for a gear, just what it acts like. Then top speed is on 10 mph for a bit then picks back up but still will not rev over 3500 rpm still bucking. I do know that it's not the catalytic converter. Any more ideas.
I'm pretty sure that is probably a computer controlled throttle. If the engine detects a significant problem, it could be in "limp" mode, cutting power so you don't harm the engine.
P0449 is EVAP system related - it's quite common on those for the EVAP vent canister solenoid to fail so that would be my prime suspect. Especially if you are experiencing any roughness at idle or if the car seems to be struggling to start.
The limited revs/acceleration sounds very much like the error code has tripped the engine into limp mode but I can't be certain.
If the problem is the vent solenoid the good news is that it's not a particularly expensive part (it's ~ $120), the bad news is that it's located on top of the fuel tank so it's a tank-out job to swap it. Not the easiest one to do at home!