I understand the issue is outside, but you commented that you could do it inside, but not replicate it outside. There have been a number of people comment that their inside and outside performances don’t line up and then you find out that their equipment is different. So, just looking to eliminate that as a factor, since you do get variation with different equipment.
What is the terrain like that you are riding on outdoors? That obviously will also affect things a lot.
I used to have difficulty holding a constant power outdoors on pan flat terrain, regardless of zone. I found that I when I was looking at the PM, I’d be over and then ease off and then I’d be under. The yo-yo effect really zapped my legs after a while. Indoors I didn’t have to think about it, the trainer just holds me in the right place in erg. So, I changed by computer to not smooth, just show the current power. I figured the smoothing was contributing to the yo-yo. Then I rode to about where it needed to be (10-20 secs of power monitoring), paying attention to how it felt. Then just tried to hold on to that feeling for as long as I could. I found that after a while, my power was much more consistent with this approach. I may not have been right on the power number that I was aiming for, but it was close enough. So, maybe your 10 sec smoothing is adversely affecting things for you?