My Mercedes CLK430 did that as the MAF sensor was going out. The easiest way to figure it out was to disconnect the battery for a few hours to reset the computer. Once the computer was reset and the engine was not running off the ECU's cache and was taking new readings it was immediately obvious which was bad as it wouldn't provide the ECU with information. Most OBDII cars learn your driving style and the ECU cache's info so when you clear it it can't reference the old settings it figures are already there as a baseline.

It's also the cause of the famous line "this wasn't like this before I changed the battery" as faulty products become instantly obvious when the ECU is reset.