[sefiro]

Jumping in late....
You mentioned that the CEL goes off when you let off the gas. A CEL code typically won't reset until a number of drive cycles or types of a drive cycle are finished.

If the car is dropping rpms, the ECU will either
A) start receivingv weird signals from the sensors
B) think it's in a starting mode, and do a dashboard light test.

The heavy exhaust smell suggests there is too much fuel it there is unburnt fuel. Either of these could cause the engine to not perform well and could lead to low rpms.