Same for me. it’s such a blow to the ego when you realize that your Tacx Vortex was so off. I thought my 20 minute power was nearing 300. 
All my outdoor FTP tests were total failures because I was nowhere near the power I thought I should be at thus I was starting off way too hot.
If you look at the “Stages vs the world” topic I started, my Vortex sometimes could be off by 10 watts, then 20 watts, then 30, 50, or even 70 watts on a standing hard effort. My Vortex would also drift. The more it was warmed up, the more it would be off from the Stages and Powertap.