Here are some more suggestions, since it seemed to work normally for a day or so and then went back to crap -

the ECU - if it occasionally messes up, it could toss lots more gas than is needed, flooding the cylinders and causing those symptoms...

Head is cracked, letting oil into the cylinder... when you started it the other day, the metal had gone back to it's normal shape, but after a day of running, it had heated up, expanded, and the crack came back? That's a longshot, but possible. Has the car overheated recently? And what does the smoke pouring out the back look/smell like? Oil? Coolant?

You should test the compression, the oil and coolant levels, and the ECU again to check for codes. Also, does anyone know if an always-open EGR valve would do anything like this? I'm pulling at straws...