TACX Power Meter Problem?

Hi, I bought a TACX Flux S in September 2019 and used it with Zwift through the following winter without any detected issues

In October 2020 I moved the trainer to my garage, in to a room I had created in order to move it out of the house. Initially I was using Kinomaps to compare experience to using Zwift. After a while I got bored with the concept and switched back to Zwift.

I immediately detected a fall off in my performance compared to the winter before. This was noticeable on any hill section . I recalibrated the trainer and this seemed to fix the problem, I then got in to a habit of calibrating before any session . Yesterday though, 24th Jan, I noticed the same problem occurred again.and this was immediately after calibrating. On this particular route there is a 2.4 mile segment with an average incline of 3.4%. On all but todays ride (and 14 dec) I average between 140 and 160 watts. On the two faulty rides the average is 85 watts !! To my uneducated eye I suspect a fault with the power meter and I have emailed Garmin for their advice.

Strava screenshots of this segment on Zwift below. This highlights what I have said above. Any thoughts on anything I can do to rectify this problem?

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