As Bill Parcels used to say, "You are what your record is". If you have been testing it recently, and it is calibrated, then that if your power output. I would take your multiple readings from the PT (again - assuming it is calibrated) more than one test on a trainer. I would look more towards changes in sleep, diet, and potentially over-training as reasons, not wind/road conditions (why a power meter is more effective than HR/Avg Speed combo for determining effectiveness.