'07 S40 White Smoke and "Slow Down or Shift up" warning

[DJSorbitron]

Replaced a thermostat housing, refilled and bled the coolant, and did an oil change. Started car up after and it ran like a champ for about 5 minutes, before shaking and sputtering, having a flashing CEL, white smoke from the exhaust, and "Slow down or Shift up" warning. What could be the issue? (there's an ongoing oil leak behind the engine, above the pan and below the exhaust). No oil seems to be mixing with the coolant and vice versa. Read some codes and got P0300 (random misfires). Car drives like shit at low RPMs (white smoke, flashing warning lights and sputtering), but drives normally otherwise. Any ideas?