Well the coolant temp sensor gives a reading to the ECU for cold start etc. It would make sense that this sensor could be giving a cold reading all the time so it works ok when the engine is cold but when it warms up it is still giving a cold reading, thus making the ecu control the fuelling all wrong.

There should be two on your engine, one which gives a reading to the warning light on the dash and one that gives a reading to the ecu. Could be worth a try!