I have recognized unexpected power/resistance fluctuations in SIM mode. On flat segments or if it slightly goes downhill, often the resistance increases a lot, without obvious reason, although in these moments the displayed gradient does not change. It is pretty unpleasant if you try to keep a steady pace and cadence. I am not sure if this is an issue of Zwift and/or the used smart trainer - I am using an Elite Direto.
@Peter_Castrop Exacly! It feels good not to be alone with this problem 
True. But I would feel a lot better if I didn’t have this problem at all.
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@Peter_Castrop: isn’t this related with Zwift now sending weight data over Ant+ FE-C to the trainer and the trainer simulate the gradient based on your weight. So if you are heavier you will have more resistance on the hills.
Do you pair your trainer as ant+ FE-C
Thanks Gerrie . After I bought the Direto in December 2017 I am zwifting on the Ipad pairing with Bluetooth. That always worked fine, until a few weeks ago. I never changed the settings. After the problem started I have tried all options like Bluetooth on Ipad, IPhone, Macbook and Windows laptop. Also ANT+ on Macbook and Windows laptop and IMac. Nothing solves the problem. Only the beta Zwift on my Android Phone connecring with Bluetooth works fine, as it did before.
And whatever my weight is, a gradiënt of -1% should never feel like climbing.
I am using Apple TV 4k, which does not support Ant+, so it cannot be related to the weight data via Ant+. Apart from that, this issue does not occur on steep climbs, but on flat or almost flat road segments.