Could be ECM but I would think unlikely if no codes. Also if no codes then I would think that sensors are OK.

However there is a fuel pressure regulator located under the silver fuel rail that runs across the front of the engine. This can cause problems like yours without setting codes. If you can get a pressure gauge on the the out line it should be about 10-15 psi as I recall. Mine was pushing 100psi and had poor MPG and fluctuating RPM at idle, but not as bad as you descibe. You may not want to get into replacement as removing the fuel rail means getting into fuel injector seals and such, but not to expensive or time involved at dealer.

Also to be considered, but I would say less likely is speed sensor (you have to remove battery to get to this bear and remove from tranny).

Also you did not mention replacing fuel filter... low odds there but worth a shot depending on miles (located near passenger side rear)

Good luck, let us know what happens!