Wild Power Fluctuations

Hi there, I’m relatively new to Zwift, having been riding for a few months. Until today I’ve always done free rides but today decided to do an FTP test. My power usually fluctuates a lot but it has never been too much of an issue for just riding around Watopia and the other worlds. However, for my FTP test (short version) I needed to put out 20 minutes of hard effort and found it impossible to measure due to the power fluctuations I am seeing. I have seen other posts where people talk about maybe 30-40W fluctuations but mine are even more extreme. Towards the end of the FTP test my power was flickering between 40-50W and 250+ watts. In the end, the test gave me an FTP of 167W, down from the 200W as was calculated previously from my free rides. It definitely feels like I am putting down more power than what I am being credited for but the way my power numbers fluctuate makes it impossible to know and to be consistent within and between rides. Does anyone know what might be going on here?

For reference, I am using a Wahoo KICKR Snap trainer. I always do a spindown using the Wahoo App before rides (after a 10-minute warm-up) and have run a couple of advanced spindowns over the last couple of months too. Today’s spindown was 14.7 seconds so within the recommended 9-16 second range.

And here’s the graph from today’s FTP test:

Thanks, Harry