OMG give this guy a friggin trophy! For once someone goes and actually checks the codes on their ECU!

Yeah, coolant temp sensors will make the ECU go crazy. There are dozens of compensation tables in the ECU that are based on engine temp. When they're pegged high or low, the engine runs like crap.

The sensor changes resistance based on temp. Unplug the sensor and measure the resistance across it (car should be off). It should be 2.1k-2.9k when the engine is cold, and about 300-330 ohms when it's at operating temp. Basically, it should be between those numbers. When they go bad you'll either get an open or short circuit.