For example, I set 200 watts in the Tacx app yesterday and I’d end up with 160 watts. I had to keep down shifting to a really high gear to get up to 200 watts (like 52x15).
I’ve never had this happen before. In the past 300 watts in a lower gear wasn’t a problem.
Is my trainer on the way out?
BTW, this is a Tacx Vortex Smart.
Use the tacx app to calibrate it.
Had similar effect with Tacx Flow Smart (basically pretty similar model to yours) - wattage was dropping or even fading to 0 occasionally, gear switch sometimes helped.
I think that was due to overheating, as it happen usually far in the training and/or during longer intervals.
Was fortunate enough to find it during warranty period and returned the trainer. Now on direct one, Elite Suito, and couldnt be bappier (and quieter!).
The trainer was calibrated.
I’m going to be pissed if a $500 three year old trainer that has hardly been ridden is on it’s last legs!
Maybe I’ll go back to my 25 year old Minoura Mag trainer that is hanging in the garage. 
I have a power meter now.
This is an interesting video:
It makes me question all my power test numbers over the last three years. First it’s interesting that the trainer has a natural power curve due to the magnets. Power above the curve is brought on by electro magnets.
in the video the guy “hacks” the trainer by loosening the tension and deflating the tire just to see the result. He gained 80 watts in FTP!
The other interesting point in the video is that the trainer needs a warmup before calibration to be as accurate as it can be.
I haven’t done any more detective work on my trainer. I’m going to next update the app, check for firmware, and do some tests.
I wanted to post in case anyone else comes along with a similar problem in the future.
I updated the firmware on my Tacx Vortex and all is working well again.
Why it stopped working well is a mystery. I’m guessing that the app was updated and needed the latest firmware on the trainer.