This is a problem that I’ve solved and this post is to help others who have the same issue.
My issue was that Zwift was stuttering in Windows and dropping my watts to zero for between 5 and 15 seconds. The graphics would also lock up in Zwift when this was happening. After a few seconds things would be fine and I would have to sprint to catch up with the pack.
Equipment: Alienware M15 gaming laptop and Stages Bike SB20
I tried absolutely everything that I could find in different guides and forums: Different graphics settings, moving the wifi router, reinstalling Windows and so on…
My problem turned out to be Bluetooth on the laptop. When I switched to using ANT+ (with an ANT+ stick in the USB port) the stuttering stops. What also fixes the problem is using the Zwift Companion app and having that send data to Zwift instead of using Bluetooth.