Maybe you're pushing harder on long flats than long climbs. What's your general HR for both? I tend to take it easier on long climbs and push harder on flats.

As for fatigue, I always want to think "fatigue" is a lack of endurance or strength, but really it's generally onset of blood lactate accumulation. (Vividly brought home to me yesterday during my lactate testing round... my ear is still sore. )

as for Spinervals - you're Troy's Queen, so if you don't know....