I was doing power-based intervals on the rollers Tuesday night, and had the following issue. NOTE: I've experienced this a handful of times before, as well. But, it doesn't happen very often, and definitely not every time I do intervals.
I was doing 10x3min with a specific power target. So, during the work/rest intervals, I was simply switching between a harder (work) and easier (recovery) gear.
For the first 3 or 4 intervals, there was no issue. Switching between the two gears, the power levels (and speeds) were consistent, as you would expect. However, the next interval, when I returned to that same harder gear and cadence, the reported power was noticeably lower (on the order of 15-20W). I actually had to shift to one gear harder (verified by the accompanying change in speed) to get to the power level that I was targeting. I finished that interval, shifted back to the recovery gear, and it, too, was reading noticeably lower than it had previously, despite the speed being the same as it had been on prior intervals. (All that to say that it wasn't just an issue of me mis-counting the shifts between the work/recovery gears.)
At that point, I got off the bike and started messing with the Garmin calibration and things got confusing. But, when I've experienced this same issue in the past, I would just keep riding and, sometimes, the reported power level would return to the (higher) initial values on subsequent intervals.
Has anyone experienced anything like this?
Like I said, it's not very often that it happens. But, when it does, it's damned annoying!
I usually just forget about it after a couple of days of it working fine, but I did remember to send their customer service a message this time. We'll see what they say...
"If you ain't first, you're last." Reese Bobby Talladega Nights