Connect the Tacx via ANT+ not Bluetooth. There is an issue with Bluetooth at the moment. Also try running in the small chain ring and the slightly larger side of the middle of the cassette. If your recovery power needs to be below 90 watts, maybe just run it in the biggest cog on the back.
Those big drop outs are typically a Bluetooth issue.
The cadence sensor on the Flux is OK but having a dedicated one is better. The Flux calculates rather than measures cadence