Hey @mackay46!
When you are riding outside your power will never hold constant like it does on the trainer. It will fluctuate some, especially if you are riding on hilly terrain, but as long as you are staying within 84-97% of your FTP and your normalized power is falling within that range for your interval you are #FtFPing. If you are having surges well above or below that range, look at the route you are riding and see if you can set yourself up better to execute the intervals by finding a road with a steady grade and fewer turns.
As far as the trainer goes, the FasCat coaches actually advise against using ERG mode for intervals as it is unrealistic to doing those efforts the outside world.
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