I have a very similar issue with the Wahoo Kickr that I’ve been battling for months - you can read about it at Bug: ERG mode has intermittent drops in resistance and reported power (Wahoo Kickr) - essentially the problem with all these units seems to be that they attempt to detect the power using mathematical algorithms based on chain tension etc, rather than using a more direct means such as a strain gauge that is fitted to power meter pedals and crank-based power meters for instance.
As a result, sometimes they simply give duff info.
I’ve not found a solution. You can definitely improve things by not “getting ahead” of the flywheel as Trevor says - i.e. using the small ring at the front and the big ring at the back (or equivalent for Stages gearing) but its not a perfect solution.
Its frustrating but its a fundamental flaw in how the trainers calculate power. I’m only really surprised that those such as GP Lama etc haven’t called the trainer manufacturers out on it as a whole. its possible that trainers that do not use a flywheel such as the Tacx (electromagnetics only) may be less susceptible but that’s a hunch and not something I’ve tested for myself. The only 100% solution would be using power meter pedals or crank power meter and just resigning yourself to not having ERG mode.